I workaround I've used is to create a dimension group of the fields:
Then just have ONE expression, but do it like this:
Now you just need to get your dimension group onto your chart where you'd normally see the expression group or wherever else you want it. I think sometimes it works if you just add the dimension group as another dimension. But you can also create some other object that has absolutely no purpose except to display the dimension group. I can figure out how I've hacked it together if you can't work it out from there.
If I remember correctly, my dimension group object is usually NOTHING but the little circular icon. I position it in the chart title, and make the chart title =' ' & getcurrentfield("My Group"), left justified, with enough spaces to make room for the icon.
Edit: OK, I found one of my applications that does this. I created a bar chart with the group as the dimension, no expression, don't display the caption, and changed the "all expressions disabled" error message to blank. That leaves you with nothing but the little icon to select the "expression" for your chart. Shrink the size of the window to barely show the icon, put it on the top layer, and drop it anywhere on your chart that you want it.
Very interesting approach for a small layout issue.
I'm afraid it will not help on a scatter chart because I need one dimension which is city, two expressions x, y for longitude and latitude and a third expression group with several expressions.
I played around with your suggestion but then I have a legend with many values and other problems. But, I need the caption of the expression only.
It turned out that the chart didn't want to display the legend for the fourth expression. To convince it to display the legend, I had to include it in the value by using dual(). I'm not sure if that made much sense, but if you REMOVE the dual(), and look at the pop up text, you should see the problem it's solving.