JEREMY BROWN uses global fields and ExecuteSQL to create context free, dynamic charts:
Charts are great visual tools, but they can quickly get messy and difficult to read. A chart that compares sales of different products throughout the year would get very unreadable if all the products were listed for all the months of the year.
Instead, we can dynamically add or subtract data series from the graph to make a better comparison. For example, I could compare apple sales to orange sales, or apple sales to banana sales, or even lime sales to strawberry sales. Or I could compare four of the products to each other. Using ExecuteSQL and a global field, a graph can be dynamic and interactive.
Well worth the read.
More...Dynamic Charting in FileMaker.