Add ggplot2 column chart examples from Sascha

* stacked
* grouped

Change-Id: I5e2a2afdbfbee5a514ce41fddbef7b2e49098ed7
diff --git a/man/figures/grouped_bars.png b/man/figures/grouped_bars.png
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diff --git a/man/figures/stacked_bars.png b/man/figures/stacked_bars.png
new file mode 100644
index 0000000..53db39c
--- /dev/null
+++ b/man/figures/stacked_bars.png
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