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

I know you can change the colors, but I don't think you can change the text because it is based on the series you select. 

I'd recommend using a chart "description" and show that on the report.  
Descriptive axis labels help, and chart titles as well to tell the whole story.

thanks Matt. I have that setup now but was hoping we could do a bit more because our graph trends are bit complex because we are comparing things that aren't similar. 

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.