Fix stacked bars example and ggsave commands in Readme.md

Change-Id: Ia37a93a7a29c2f41f15fa9c7f6a7da5073825f61
diff --git a/man/figures/stacked_bars.png b/man/figures/stacked_bars.png
index 53db39c..f55126b 100644
--- a/man/figures/stacked_bars.png
+++ b/man/figures/stacked_bars.png
Binary files differ