Set data-dependent height for bar chart

[erxclau]

Currently, bar charts have a default height of 150. I think a better default would be to make height some multiple of the number of keys that the data has.

Say we want each bar to be 30 pixels in height. If we have 10 data entries (i.e. 10 bars), then the height of the bar chart should be set to 300 pixels (in reality the bars will be less than 30 pixels because of scaleBand padding but the idea stands).

If someone wants to set a specific height, then they'd still be able to do so with the height setter.