a graph for every report

Not yet perfect; they're all pie charts, but some should be bar
charts. Also, the way I exclude the total from the chart is a bit of a
hack.
don't show the chart if there's no data to render
track paths directly, rather than just the order
fix graphs when the graphed values are a little silly
add 'other' column showing answers that don't fit into the predefined categories

Fixes #48 for all reports except the ones that can only count how many
are answered, which will be fixed a different way.
Modified tests for new graphs and charts.
greatly simplify reporing queries

This makes the query readable and understandable, and enables us to do
a coverage report. Use the coverage report on all questions where no
other type of report is currently feasible.

Fixes #37 and #48
show the coverage report for every question

This means that most questions show two tables and two graphs.
tweak how we get the legend colors
add coverage report to question 15
replace the reporting query with a more correct version

It's not quite as simple, but it has the virtue of couning everything
correctly.
tweak the styling so that an empty graph doesn't cause the charts to misalign