Legend problems in MatPlotLib

John Hunter jdhunter at ace.bsd.uchicago.edu
Thu Dec 16 17:52:10 CET 2004

>>>>> "Jorl" == Jorl Shefner <v451v at yahoo.com> writes:

    Jorl>    I've only been able to plot data with both symbols and
    Jorl> lines by issuing two plot commands, one for markers and one
    Jorl> for lines.  That's perfectly fine, but it creates a problem
    Jorl> when I try to create a legend for it.  For some reason, the
    Jorl> legend command by default alternates between using symbols
    Jorl> and lines, grabbing displaying the symbol from the first
    Jorl> plot command for the first series, then displaying a line
    Jorl> type from the second plot for the next series, etc.  This
    Jorl> behavior is a bit strange, and in fact unacceptable when the
    Jorl> same line type is used for each series.

Example code always helps, but note you can plot a line with lines and
markers like this

  plot(x, y, '--s')  # dashed line with square markers.

In this case matplotlib will recognize this as one line and should do
the legending properly.  

However, if you do, which I infer you are from your post

  plot(x, y, 's')  # square markers.
  plot(x, y, '--s')  # square markers.

the plot will look the same but matplotlib considers this to be two
different lines and the legend will be messed up.

Note you can explicitly control what gets added to the legend, eg with

  l1, l2 = plot(x, y, '--s', x1, y1, 'go')  
  p = bar(x3,y2)
  legend((l1, p), ('My lines', 'My bars'))

In other words, if you don't like the way the autolegend is setup, you
can explicitly control the process.

Hope this helps.


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