4.1 Introduction

PyX can be used for data and function plotting. At present x-y-graphs and x-y-z-graphs are supported only. However, the component architecture of the graph system described in section 4.2 allows for additional graph geometries while reusing most of the existing components.

Creating a graph splits into two basic steps. First you have to create a graph instance. The most simple form would look like:

from pyx import *
g = graph.graphxy(width=8)

The graph instance g created in this example can then be used to actually plot something into the graph. Suppose you have some data in a file graph.dat you want to plot. The content of the file could look like:

To plot these data into the graph g you must perform:

g.plot(graph.data.file("graph.dat", x=1, y=2))

The method plot() takes the data to be plotted and optionally a list of graph styles to be used to plot the data. When no styles are provided, a default style defined by the data instance is used. For data read from a file by an instance of graph.data.file, the default are symbols. When instantiating graph.data.file, you not only specify the file name, but also a mapping from columns to axis names and other information the styles might make use of (e.g. data for error bars to be used by the errorbar style).

While the graph is already created by that, we still need to perform a write of the result into a file. Since the graph instance is a canvas, we can just call its writeEPSfile() method.

g.writeEPSfile("graph")

The result graph.eps is shown in figure 4.1.

**Figure:** A minimalistic plot for the data from file graph.dat.
$\includegraphics{graph}$

Instead of plotting data from a file, other data source are available as well. For example function data is created and placed into plot() by the following line:

g.plot(graph.data.function("y(x)=x**2"))

You can plot different data in a single graph by calling plot() several times before writeEPSfile() or writePDFfile(). Note that a calling plot() will fail once a graph was forced to ``finish'' itself. This happens automatically, when the graph is written to a file. Thus it is not an option to call plot() after writeEPSfile() or writePDFfile(). The topic of the finalization of a graph is addressed in more detail in section 4.3. As you can see in figure 4.2, a function is plotted as a line by default.

**Figure 4.2:** Plotting data from a file together with a function.
$\includegraphics{graph2}$

While the axes ranges got adjusted automatically in the previous example, they might be fixed by keyword options in axes constructors. Plotting only a function will need such a setting at least in the variable coordinate. The following code also shows how to set a logathmic axis in y-direction:

from pyx import *
g = graph.graphxy(width=8, x=graph.axis.linear(min=-5, max=5),
                           y=graph.axis.logarithmic())
g.plot(graph.data.function("y(x)=exp(x)"))
g.writeEPSfile("graph3")
g.writePDFfile("graph3")

The result is shown in figure 4.3.

**Figure 4.3:** Plotting a function for a given axis range and use a logarithmic y-axis.
$\includegraphics{graph3}$