How to fill an areaΒΆ

In this example, we show how to fill specified area with a given color.

[1]:

import openturns as ot

We generate a sample from a standard gaussian distribution.

[2]:

dist = ot.Normal()
dist.drawPDF()

[2]:
../../_images/examples_graphs_graphs_fill_area_4_0.png 
[3]:

def linearSample(xmin,xmax,npoints):
    '''Returns a sample created from a regular grid
    from xmin to xmax with npoints points.'''
    step = (xmax-xmin)/(npoints-1)
    rg = ot.RegularGrid(xmin, step, npoints)
    vertices = rg.getVertices()
    return vertices

[4]:

a = 1.
b = 2.

[5]:

nplot = 100 # Number of points in the plot
x = linearSample(a,b,nplot)
y = dist.computePDF(x)

The following function uses the PolygonArray class to create a area filled with a given color.

[6]:

def drawInTheBounds(vLow,vUp,n_test):
    '''
    Draw the area within the bounds.
    '''
    palette = ot.Drawable.BuildDefaultPalette(2)
    myPaletteColor = palette[1]
    polyData = [[vLow[i], vLow[i+1], vUp[i+1], vUp[i]] for i in range(n_test-1)]
    polygonList = [ot.Polygon(polyData[i], myPaletteColor, myPaletteColor) for i in range(n_test-1)]
    boundsPoly = ot.PolygonArray(polygonList)
    return boundsPoly

Compute the bounds to fill: the lower vertical bound is zero and the upper vertical bound is the PDF.

[7]:

vLow = [[x[i,0],0.] for i in range(nplot)]
vUp = [[x[i,0],y[i,0]] for i in range(nplot)]

[8]:

area = dist.computeCDF(b) - dist.computeCDF(a)

[9]:

boundsPoly = drawInTheBounds(vLow,vUp,nplot)
graph = dist.drawPDF()
graph.add(boundsPoly)
graph.setTitle("Area = %.3f" % (area))
graph.setLegends([""])
graph

[9]:
../../_images/examples_graphs_graphs_fill_area_13_0.png

The CDF difference is equal to the area under the curve.