The Graph and Curve Objects

1. Introduction

In this chapter, the objects for performing basic x/y-plotting, are described. x/y-plotting consists of three steps: (1) Obtain the x- and y-data, (2) put those data into one or more Curve objects and put those objects into a Graph object. Finally, make adjustments to the display settings of the Graph object and of the Curve objects.

Here, we are concerned with steps 2 and 3. Note that, for CSM (computational solid mechanics) and CFD (computational fluid mechanics), the specialized HistoryXYPlot and DPTools objects perform all three steps for many standard tasks. Further, these objects have a GUI, while at least for the Curve object, some Python scripting is necessary.

However, these specialized objects also make use of the Curve and Graph objects to display the x/y-data.

The following classes are of relevance to x/y-plotting:

  • The Graph object is a container for one or more Curve objects. Like a Part, it can be displayed in a Scene. The Graph object consists of an x- and a y-axis, for which start and endpoints, the number of intervals, sub-intervals, etc. can be specified or left to the Graph object to determine automatically (the default). The axes scales can be inverted, and logarithmic scaling for the axes is also possible. Finally, a host of display settings is available.

  • The Curve object is a container for a list of pairs (x,y). It also stores attributes, like colour, line width, symbol type and size etc. With this object, scatter, line, and bar plots, or a combination of those, can be made. For each Curve object, a label (a character string) can be defined. The Curve labels are displayed together in the Graph's area.

The following simple example demonstrates the use of several Curve objects in the same Graph object.

# Define data ranges.
x1 = [-1.0 + 2.0 / 100 * i for i in range(101)]
y1 = [x**2 for x in x1]

x2 = [-1.0 + 2.0 / 20 * i for i in range(21)]
y2 = [x**3 for x in x2]

# Create two Curve objects and initialize them.
c1 = Curve()
c1.extend(zip(x1, y1))
c1.label  = 'x^2'

c2 = Curve()
c2.extend(zip(x2, y2))
c2.colour      = 'blue'
c2.symbol.type = 'cross'
c2.label       = 'x^3'

# Create the Graph object and add the Curve objects.
g = Graph()
g.xaxis.label = 'x'
g.yaxis.label = 'y'

When executing this script, the Graph object automatically computes the start and end values for the x- and y-axes.