rqt_plot: decreased required minimum padding to 0

[forrestv]

With a fixed absolute minimum padding, rqt_plot is useless for plotting
arbitrarily small values. (For example, trying to plot a magnetic field
measured in teslas, usually having a magnitude of about 1e-4.)

[dirk-thomas]

This patch results in a delta potentially being zero which triggers the following warning for me:

UserWarning: Attempting to set identical bottom==top results in singular transformations; automatically expanding.
bottom=VALUE, top=VALUE

[dirk-thomas]

Your patch could do the same thing matplotlib does internally to avoid zero size ranges:

bottom, top = mtransforms.nonsingular(bottom, top, increasing=False)

[DorianScholz]

@forrestv: I agree with getting rid of the fixed minimum padding in the matplotlib backend, maybe in the way Dirk suggested.
But besides that, have you tried using the much preferred pyqtgraph backend instead of matplotlib:

• install deb from http://www.pyqtgraph.org/
• select backend from rqt_plot settings dialog

This should also avoid this issue because this padding is specific to the matplotlib backend.
Nevertheless I really appreciate your work on this and will incorporate your fix, if the issue with the zero delta warning is resolved.

[forrestv]

Okay, I fixed the warning by adding a fixed amount of padding, only if the data's range has zero width.

Also, in a separate commit, I fixed adding more and more padding as more curves (topics) are added, which was probably unintended.

[dirk-thomas]

This will fail when being computed outside of the loop since ymin etc. might be None.

[forrestv]

Oops. Fixed now.