How do I change height of graph without making it look distorted?

Plugin WordPress Charts:

My WP graph looks like this in the screen version of my website:
http://i59.tinypic.com/1zvl92.png

For the print version I want only half of the height. When I change the height of the canvas box then I get a distorted graph:
http://i57.tinypic.com/mii8sl.png

Anybody has an idea how I can make the graph smaller regarding height with the bars getting smaller, but WITHOUT the chart numbers getting squeezed?

https://wordpress.org/plugins/wp-charts/