New plotstyle filledlines

[goettlic]

Add new plottype filled lines, some adjustments in changing the labels of the used widgets, adding zerolinecolor to make the x- and y-axes visible (also in scatter plot). Refers to #393

[ghtmtt]

fantastic! Will review ASAP and merge

[ghtmtt]

some comments:

• is it possible to add a main line color and a filled line color? From https://plotly.com/python/line-charts/#filled-lines is might be useful to separate them
• interactivity between plot and features is not enabled, it is on purpose? Else the place to add it is here https://github.com/ghtmtt/DataPlotly/blob/master/DataPlotly/core/plot_factory.py#L513

else, great work!

[goettlic]

Thank you for your comments.

• is it possible to add a main line color and a filled line color? From https://plotly.com/python/line-charts/#filled-lines is might be useful to separate them

In this version of the new plotstyle there is no single line drawing at all, just the filled polygon between the two data entries for the upper and the lower boundary and therefore only the possibility to select a color for the fill (but this with a transparent option). If we want to have a line in the middle of the polygon or for the border of the filled part we have to make an additional plot with "scatterline" where there is the option for the line color as well.
In a first version I tried to append a filled line to an existing scatterline, but that gave some strange behavior when updating the existing plot because two traces where drawn in a single event. If we want a single line with an optional filled polygon underneath in a single step I have to have a deeper look into how the line is actually drawn and make new widgets to enter the upper and lower y-values. In that case it would be necessary to split the color selection as well. In the moment the same color widget ("in_color") is used for different things (marker color of points, line color of lines and now fill color for filled lines, see #220 as well).

• interactivity between plot and features is not enabled, it is on purpose? Else the place to add it is here https://github.com/ghtmtt/DataPlotly/blob/master/DataPlotly/core/plot_factory.py#L513

I tried to keep it simple with no hover options but did not have the interactivity on my sight. The idea was more of an error area for an existing scatterline so no need for the interactivity. On the other side I tested with timeseries data: more data values for only one feature. I think, to make usefull lines in a scatterplot with multiple features it would be necessary to have the option to sort over the x-values.

Any suggestions very much appreciated!