Is it possible to create a custom legend on a bar graph?

Using script you can modify a QuickBase rendered chart an change any options including the legend. Here are two examples that (1) changed the Yaxis and (2) change the data to cumulative:

https://haversineconsulting.quickbase.com/db/bk89redf6?a=q&qid=1

The key to doing this is (1) to get a programmatic handle to the chart, (2) extract the current options, (3) destroy the current chart, (4) modify the original chart's options, and (5) redraw the chart with the new options:

var chart = Highcharts.charts[0];
var options = chart.options;
chart.destroy();
options.yAxis[0].min = 0;
options.yAxis[0].max = 80;
new Highcharts.Chart(options);

In the above examples I am animating the full process from a tasks table. However, if you embed a chart in a report the process can be made seamless so only the final chart appears.