Spider Plot Index Memory

[morganjwilliams]

Is your feature request related to a problem? Please describe.
Currently, spider plots do not take into account previous spider plots which may have been added to an axis. Repeated calls to pyroplot.spider or pyrolite.plot.spider will thus i) reset the x-axis ticks and ticklabels to the new index (erasing records of the old) and ii) ignore the index ordering of the previous plots, such that they are likely to have different ordering.

In the meantime, if you wish to plot over existing spider plots, consider reindexing dataframes such that they're consistent throughout.

Describe the solution you'd like
Keeping a memory of previous spider plots, and using this to generate a new composite index (including all items from old and new series, and preserving old ordering for duplicated items).

Describe alternatives you've considered
This could be achieved through checking the x-axis tick labels or adding attributes to the axis directly (or through subclassing the axes, but this is unnecessary at this stage).