Fixes for Lets-Plot on the Common Plots page

[ASmirnov-HORIS]

I only make some updates connected to Lets-Plot in the vis-common-plots.ipynb notebook:

• Issue #44: geom_segment() instead of geom_path() in the connected-scatter-plot section.

• Issue #45: joint_plot() instead of ggmarginal() in the marginal-histograms section.

• Issue #46: geom_area() instead of geom_freqpoly() in the overlapping-area-plot section.

• Issue #47: Add geom_area_ridges() to the ridge-or-joy-plots section.

• Issue #48: geom_contourf() instead of geom_contour() in the contour-plot section.

• Issue #49: In the pyramid section: to improve tooltips displaying I suggest not to use identity statistic, but calculate and add weight for users.

[aeturrell]

Wow, this looks really exciting. Will take a proper look as soon as I can, but my prior is that if it improves the look of the plots then it would be good to put through. Thanks a lot.

[aeturrell]

Thank you so much for this @ASmirnov-HORIS, great improvements!