Re: [Matplotlib-users] Scatter legend with colored marker

On Wed, Aug 4, 2010 at 10:00 AM, Jorge Scandaliaris
<jor...@ya...>wrote:

> Jorge Scandaliaris <jorgesmbox-ml@...> writes:
> >
> > Hi,
> > I am adding several scatter plots to the same axis, each having a
> specific
> > color. When I call legend on the axis it correctly picks all scatter
> plots with
> > their symbols and labels, but it doesn't pick up the color. The example
> below
> > demonstrates this. What I would like to do is to have in the legend the
> marker
> > its color matching that of the set it represents. How would I do this?
>
> Answering my own question, I found a way to achieve what I wanted by using
> a
> proxy artist as suggested in this thread:
>
> http://thread.gmane.org/gmane.comp.python.matplotlib.general/20995/focus=20999
>
> Maybe there's a better approach. I certainly would love legend to pick up
> on the
> color used in the scatter plot by its own.
>
> Jorge
>
>
>
Hmm, this definitely looks like a bug.  If I explicitly state what color I
want using the "color" keyword instead of using 'c' and 'norm', then
everything works properly.

Digging through the code, 'color' gets used right after the creation of the
collection object by calling the .update() function, which in turn calls the
.set_color() function which calls the set_facecolor() and set_edgecolor()
functions.  However, when using 'c' and 'norm', and having 'c' contain an
array that is the same length as the data, causes colors to be set to None
during the creation of the polygon collection.  After the collection
creation, set_array(), set_cmap() and set_norm() are called, which allows
for the scatter graph to be colored correctly upon draw.

However, I suspect that when the legend is being created, the legend uses
whatever the face/edge color that was set for the collection, which probably
isn't set yet and won't be until a draw is performed.

Ben Root