revisit legend and national plot labelling for clarity


The national plot has previously been labelled for each area section, and has a corresponding legend at the bottom showing the percentile breaks for each category. This needs to be reviewed to keep in mind:
- repetitive labelling between percentile legend and chart. is it confusing?
- double y-axis for total proportion of gages and each flow category. is it confusing?
- language about flow percentiles
- in the legend `even.steps = FALSE` but the appearance is that `even.steps = TRUE`
- wet and dry spectrum vs each categorical label.

Start with the national plot and its legend shown in the attached flow_cartogram image. Review the repeated labels, dual y-axis, flow-percentile wording, even.steps appearance, and wet/dry spectrum labels, then define the desired presentation and verify the resulting chart is clearer.

Written by the indexing model from the issue text.