Specify a start value for the y-axis of a line graph?

Is there a way to make sure the y-axis of a line graph starts at 0? It's an important option for proper line graphs.

For instance, in ICO, if you specify: 'start_value: 0' as an option, the y-axis will start at zero, no matter what the contents of your data series is. 

Overall, this is a great library. Great work!
