How do you decide what chart type to use for a given result?

Choose based on the comparison you want the audience to make. Time trends go on a line chart, category comparisons on a horizontal bar, part to whole on a stacked bar rather than a pie unless there are two or three slices, and relationships on a scatter. Start bar chart axes at zero, sort categories by value rather than alphabetically, and label directly instead of relying on a legend.

I start by asking what comparison I want someone to make in about three seconds, and the chart follows from that. Change over time is a line. Comparing categories is a horizontal bar, sorted by value, because alphabetical sorting hides the ranking that is the whole point. Part to whole I will do as a stacked bar unless there are literally two or three slices, since nobody can compare pie wedges accurately. Relationships between two continuous variables get a scatter. Then a few rules I hold to regardless: bars start at zero because the length is the message and truncating it exaggerates differences, labels go on the series directly rather than in a legend people have to keep looking back at, and I do not use color unless it encodes something, since decorative color is just noise. The last thing is the title. I write the finding, so mobile conversion fell 12% since March, not conversion by device over time. That one change makes people read the chart correctly.