remove adding data to plot feature

[jo-mueller]

Hi @zoccoler ,

I was playing around a bit with the color settings (see PR #18 ) and I was wondering whether it might be easier for future expansion to remove the feature of adding data? With respect to setting the alpha, size, face- or edgecolor, I find it hard to keep the values in sync when data can be concatenated.

In the concrete case, upon adding data, the plotter tracks which data points have already been set, and preserves their colors. So one would have to do the same for all other properties of the scatter plot, which I see getting out of hand a bit in the future 😬

LMKWYT :)

edit: In practice, there could be a reset() method that clears color indices, alpha & size and then redraws the plot if new data is added. For plots with a large number of datapoints where (redrawing) would make the widget slow, I guess one would turn to the histogram artist anyway.