Update index.md

Author: RajKhanke

contrib/plotting-visualization/index.md

@@ -10,4 +10,5 @@
 - [Seaborn Plotting Functions](seaborn-plotting.md)
 - [Getting started with Seaborn](seaborn-basics.md)
 - [Bar Plots in Plotly](plotly-bar-plots.md)
-- [Pie Charts in Plotly](plotly-pie-charts.md)
+- [Pie Charts in Plotly](plotly-pie-charts.md)
+- [Violin Plots in Matplotlib](matplotlib-violin_plots.md)