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Plotting data contained in a file
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This example shows how to draw a graph representing data stored in a file. ...
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We assume that the data is arranged in the file `minimal.dat` in a whitespace-separated
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The first step is to create an instance of the `graphxy` class which can be
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found in the `graph` module. By convention, we call it `g`. The constructor
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expects at least some information about the desired size of the graph. Here, we
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specify a width of 8 cm.
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! If we only specify one dimension of the graph size, PyX calculates the other
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automatically, assuming a ratio corresponding to the golden ratio.
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Next, we add some data to the yet empty graph. In order to do so, we first
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create a `graph.data.file` instance, which reads the file with the name given
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as the first argument, i.e., in the present case, `"minimal.dat"`. In addition,
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we have to specify, how the data is organized in the file. To this end, we use
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the keyword arguments `x=1` and `y=2`, which tell PyX that the first (second)
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column of the file contains the x (y) values. The `graph.data.file` instance is
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then directly passed to the `plot` method of the graph `g`.
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! Note that PyX by default ignores comments starting by a # sign when reading
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in the data from the file.
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!! The previous statement is actually not completely correct, as PyX
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uses the last comment preceding the actual data to give names to the columns.
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Thus, for a file looking like
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# my data (this line is ignored by PyX, but not the following)
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you wouldn't need to label the columns in the `graph.data.file` call at all.
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Finally, we write the graph to an EPS and PDF file. Here, we use that every
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graph is (by inheritance) an instance of the `canvas` class, as well, such that
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we can directly write it into a file.
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! Of course, you can also insert a graph into another canvas and write this
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canvas later to a file. This way, you can, for instance, easily arrange more
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than one graph on a page. Later examples will make use of this fact.
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In PyX, the way data is plotted in a graph is defined by a so-called graph
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style. A couple of standard graph styles are contained in the module
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`graph.style`. Depending on the data source, PyX chooses a default style. Here,
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we are taking the data from a file and PyX assumes that the values represent a
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discrete set of data points. Hence, it chooses the symbol style
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`graph.style.symbol` to plot the data. To override this default behaviour, you
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can pass a list of styles as second argument to the `plot` method. For instance,
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to have PyX drawing a line through the data points, you can use
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g.plot(graph.data.file("minimal.dat", x=1, y=2), [graph.style.line()])
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examples/graphs/minimal.txt
