Hi Erich, thanks for the reply.
I replicated your result for my system, and did some more tests.The result is that the behaviour is inconsistent. It will usually work as expected, but occasionally the plot randomly decides to quantize the value to an integer. It seems to happen randomly for any debug session, and I can't seem to kick it out of the condition without restarting the debug session. I am using MCU-Link, but as you mentioned that probably doesn't matter -- it does always show the correct float value in the global vars window.
Peter