Feature request: user triggered chart generation

[Molrn]

Chart generation is a very powerful feature, but it is generated for every question, despite being most of the time not necessary. It can be an issue when the app is used on large datasets, because the chart generation is then very long and consumes a lot of tokens.

The first solution used to adress this problem (VannaBase.should_generate_chart) already narrows the amount of unnecessary chart generation, but there is in my opinion a much simplier solution to this issue: make the chart generation user-triggered. This would mean adding a button Generate graph after the SQL query has succeeded.