I was not able to control the width of the chart, but was able to trick it by 1) searching into the future and/or 2) controlling the width of the legend (which can be a fake one if you don't want it to show, since it'll get cutoff)
I also have a similar issue. I'm plotting a timechart with stacked bar graphs for the
search|timechart count(event_type) by event_type
When it renders on my browser it looks fine but the report cuts off the right hand side and some of the bottom (if i add the legend on the bottom). It looks like a result table formats correctly
It resizes to the print window but maybe it's an issue with timechart?