Plotting a curve

The shortest example, the function $y = x^2$ is evaluated with values inside the range defined by the canvas size (the default dimensions are 550x350)

The required parameters are:

• target a selector to the node to hold the graph
• data an array of objects which contain info about the functions to render
• data.fn (string) a mathematical expression to render, it's parsed and evaluated using interval-arithmetic

The syntax of the string that represent the mathematical expression is just like ECMAScript however the ^ operator has been replaced with pow and there's no namespace for the functions to be evaluated

Additional options

Additionally the graph can have the following options defined on the top level object:

• title: the title of the graph
• width: width of the graph
• height: height of the graph
• xAxis:
• type: the type of scale for this axis, possible values linear|log (default value: 'linear')
• label: x axis label
• domain: x axis possible values (see examples below)
• yAxis: same options as xAxis
• disableZoom: true to disable translation/scaling on the graph

Grid

Set grid: true in the options sent to function plot

Domain

The domains of both axes can be changed with the following configurations:

• xDomain, defaults to [-7, 7]
• yDomain, keeps a 1:1 aspect ratio relative to xDomain, by default it's computed with the following formula

$$ yDiff = \frac{height * (xDomain[1] - xDomain[0])}{width} $$

NOTE: The origin is at the center of the graph by default so $yDiff$ is split in half and distributed evenly to the $\pm y$ axis

Number of samples

nSamples determine the number of equally spaced points in which the function will be evaluated in the current domain, increasing it will more accurately represent the function using rectangles at the cost of processing speed

e.g. nSamples = 100

$$ domain = [-5, 5] \ values = -5, -4.9, -4.8, \ldots, 4.8, 4.9, 5.0 $$

$$ domain = [-10, 10] \ values = -10, -9.8, -9.6, \ldots, 9.6, 9.8, 10 $$

Annotations

Parallel lines to the y-axis or x-axis can be set in the annotations option:

• x: x coordinate of a line parallel to the y-axis
• y: y coordinate of a line parallel to the x-axis
• text (optional) text shown next to the parallel line

NOTE: either x or y need to be set on the object, setting both of them will raise an exception

Range and closed path

The x values of the function will be taken from the current viewport limits, however you can define a custom range so that the function is evaluated only within this range, this works really nice with the closed option which will will render an area graph instead of a polyline, for example we can render a definite integral

• range {Array} A 2-number array, the function will be evaluated only within this range
• closed: true to render a closed path, y0 will always be 0 and y1 will be $fn(x)$

Logarithmic scales

The type of each axis can be configured to be logarithmic by specifying the type of axis to log inside the xAxis option, note how this change affects the way the functions are sampled

Multiple graphs

data as seen in the examples above is an array, which means that multiple functions can be rendered in the same graph

You can also change the color of each graph, by default the colors are set from functionPlot.globals.COLORS but you can override the color by setting the color option in each datum

Graph types

There are three ways to represent a function in function-plot

• polyline where $f(x)$ is evaluated with some $x$ values, after the evaluation the points are joined with line segments using <path>s
• scatter where $f(x)$ is evaluated with some $x$ values, after the evaluation the points are represented by <circle>s
• interval where $f(x)$ is evaluated with intervals instead of a single point, after the evaluation 2d rects are painted on the screen (done using the <path> svg element)

Set the type of graph you want to render in the option graphType (defaults to interval)

Tip

The little circle that has the x-coordinate of the mouse position is called a "tip", the following options can be configured:

• xLine true to show a dashed line parallel to $y = 0$ on the tip position
• yLine true to show a dashed line parallel to $x = 0$ on the tip position
• renderer a custom rendering function for the text shown in the tip

NOTE: the tip only works with linear functions

When you don't want a function to have a tip add the skipTip: true property to the object that holds the info of the function to render

nth-root

Plotting roots can be a challenging problem, most plotters will actually analyze expression of the type $x^{\tfrac{a}{b}}$, particularly they will analyze the denominator of the exponent (to plot in the negative x-axis), interval-arithmetic and math.js come bundled with a useful nthRoot function to solve these issues

Secants

If a data object has a secants array, then each object will be used to compute secant lines between two points belonging to the function, additionally if updateOnMouseMove is a property set to true in the object then $(x_0, f(x_0))$ will be used as an anchored point and $(x_1, f(x_1))$ will be computed dynamically based on the mouse abscissa

Available options for each object:

• x0 the abscissa of the first point
• x1 *(optional if updateOnMouseMove is set) the abscissa of the second point
• updateOnMouseMove (optional) if set to true x1 will be computed dynamically based on the current position of the mouse

Secants

Dynamic secant lines

An example where updateOnMouseMove is set in two secant lines, each line will be computed on the dynamically based on the current position of the mouse

Derivative

If a data object has a derivative object then its property fn will be used to compute the equation of the line tangent to the point x0, i.e. the point $(x_0, f(x_0))$ (the derivative is a function which gives the slope of the tangent line at any derivable point)

Available options for the derivative object:

• fn The function that is the first derivative of data.fn
• x0 (optional if updateOnMouseMove is set) the abscissa of the point belonging to the curve whose tangent will be computed
• updateOnMouseMove if set to true x1 will be computed dynamically based on the current position of the mouse

Derivative

Tangent line on mouse's position

if updateOnMouseMove is set to true then tangent line is computed whenever the mouse is moved inside the canvas (let $x_0$ be the mouse's abscissa then the tangent line to the point $(x_0, f(x_0))$ is computed whenever the position of the mouse changes)

Derivative

Multiple tangent lines

An example of a graph with multiple functions, each function is configured with a derivative object with auto update of the slope as described above

Linked graphs

Multiple graphs can be linked, when the tip's position, graph scale or graph translate properties are modified on the original graph the linked graphs are signaled with the same events, in the following example a also fires the internal all:zoom event, the zoom operation is performed on a and b, but when b fires the all:zoom event the zoom operation is only performed on b

Linked graphs

Multiple

Since the zoom event is dispatched to all the linked graphs, one can set as many linked graphs as wanted and all of them will perform the same zoom operation, in the following example these functions are plotted:

• $y = x^2$
• $y' = 2x$
• $y'' = 2$

Update

To update a graphic one needs to call functionPlot on the same target element with any object that is configured properly

Function continuity

Some functions are not defined under some range of values, for example the function $f(x) = \frac{1}{x}$ is undefined when $x = 0$, the library identifies these kind of peaks and there's no need to explicitly tell these asymptotes

Function continuity

$tan(x)$

Plotting $f(x) = tan(x)$ which has many vertical asymptotes

The tale of a circle

Consider plotting the function

$$ x^2 + y^2 = 1 $$

which is the equation of a circle of radius 1, unfortunately $y$ is not expressed in terms of $x$ and function-plot needs an equation in the form $y = f(x)$, solving for $y$ we get:

$$ y = \pm\sqrt{1 - x^2} $$

Which raises two functions

$$ y = \sqrt{1 - x^2} \quad and \quad y = -\sqrt{1 - x^2} $$

Parametric equations

The original equation of the circle $x^2 + y^2 = 1$ can be parametrized as

$$ x = cos(t) \\ y = sin(t) $$

For $0 \leq t \leq 2 \pi$

The options that tell function-plot to render a parametric equation are defined inside each term of the data array and need to have the following properties set:

• fnType: 'parametric' to mark this term as a parametric equation
• x the x-coordinate of a point to be sampled with a parameter t
• y the y-coordinate of a point to be sampled with a parameter t
• range = [0, 2 * Math.PI] the range property in parametric equations is used to determine the possible values of t, remember that the number of samples is set in the property nSamples

NOTE: function-plot uses interval-arithmetic by default, to create a nice line instead of rectangles generated by the interval-arithmetic sampler set graphType: 'polyline' which uses the normal single point evaluation

Parametric Equations

Butterfly curve

Let's render the famous equation of the butterfly curve using parametric equations, the equations are:

$$ x = sin(t)(e^{cos(t)} - 2cos(4t) - sin(\tfrac{t}{12})^5) \\ y = cos(t)(e^{cos(t)} - 2cos(4t) - sin(\tfrac{t}{12})^5) $$

Polar equations

The original equation of the circle $x^2 + y^2 = 1$ can be expressed with the following polar equation

$$ r = r_0 ; cos(\theta - \gamma) + \sqrt{a^2 -r_0^2 sin^2(\theta - \gamma)} $$

Where $\theta$ is the polar angle, $a$ is the radius of the circle with center $(r_0, \gamma)$

The options that tell function-plot to render a polar equation are defined inside each term of the data array and need to have the following properties set:

• fnType: 'polar' to tell function plot to render a polar equation
• r a polar equation in terms of theta
• range = [-Math.PI, Math.PI] the range property in polar equations is used to determine the possible values of theta, remember that the number of samples is set in the property samples

NOTE: function-plot uses interval-arithmetic by default, to create a nice line instead of rectangles generated by the interval-arithmetic sampler set graphType: 'polyline' which uses the normal single point evaluation

Polar equations

Polar rose

Rendering the equation of the polar rose

$$ r = 2 sin(4 \theta) $$

Implicit functions

The equation of a circle of radius 1 $x^2 + y^2 = 1$ expressed in an explicit way is:

$$ y = \sqrt{1 - x^2} \quad and \quad y = -\sqrt{1 - x^2} $$

This library can also plot implicit equations with the only requirement of making the equation equal to zero and adding the option implicit (the sampler expects that the function depends on the variables $x$ and $y$)

$$ 0 = x^2 + y^2 - 1 $$

To render implicit equations you have to make sure of the following:

• fn(x, y) means that the function fn needs to be expressed in terms of x and y
• fnType: 'implicit' is set on the datum that is an implicit equation

NOTE: implicit functions can only be rendered with interval-arithmetic

Implicit function

complex implicit functions

Consider the following equation

$$ cos(\pi x) = cos(\pi y) $$

It's impossible to find an explicit version of it because we would need an infinite number of functions, however for a finite region of the plane a finite number of functions suffice

Points and polylines

To plot a collection of points or a polyline the following options are required:

• points An array of coordinates, each coordinate is represented by a 2-element array
• fnType: 'points' to tell function plot that the data is already available on points

Note that you can use either scatter or polyline in the option graphType

Vectors

To render 2d vectors set the following on each datum

• vector {Array} the vector itself
• offset (optional) {Array} displacement from the origin
• fnType: 'vector' to tell functoin plot that the data is already available on vector
• graphType: 'polyline' to render a nice segment from offset to offset + vector

Text

To render text set the following on each datum

• graphType: 'text'
• location {Array} The location of the text, an array with 2 elements.
• text {string} The text to display.

Advanced: sampler

function-plot uses interval-arithmetic math by default, unfortunately some functions are not implemented yet because of the underlying complexity, for this reason you can always evaluate a function with , to do so make sure that you include math.js before function-plot

<script src="//cdnjs.cloudflare.com/ajax/libs/mathjs/1.5.2/math.min.js"></script>

And then set the following:

• sampler: 'builtIn' the parser bundled with function-plot will be replaced with the one in math.js
• graphType: 'polyline' or graphType: 'scatter'

Advanced: property evaluation

All of the examples above used a string in the property to evaluate e.g.

 // n
 functionPlot({
   data: [{
     fn: 'x^2'
   }]
 })

You can use a function instead of a string, the input will vary depending on the type of fnType

For any case the input will be a single object and its properties will be the same as the ones the function depends on, e.g. when fnType: 'polar' then the function depends on theta so theta will be a property in the input object

if you want to use any other plotter your function is expected to return a single value (commonly used)

if you want to use the interval arithmetic plotter your function is expected to return an object with the properties hi, lo (rarely used unless you want to make computations with an interval arithmetic library)