This bug seems to be around since a few versions according to a quick search. The ghost proxy icon is not reset properly after an invalid drag event.
Here is a screenshot with 4.1.1 RC2.
PS : my code is a bit too large to be posted here, but I can post it if required.
Edit1 : tested with IE7, IE8; IE9, Chrome 20.0.1132.47 (ouch)
Edit2 : sorry, not sure where to post this... I already double post it because I made a mistake, using the thread 4.1.1RC1 instead of 4.1.1 RC2.
No, I haven't created an official bug report. I didn't take the time to do it yet because it would be time consuming to fully document my scenario because my configuration is complex - I have to cover a lot of corner cases for missing data. Guess I should though...
I had to experiment a lot to figure out what to use for a missing value in a series, e.g. NaN, -1, 0, MAX_INT, etc. Some of these worked and some didn't. I didn't see any documentation that said what to do for missing data - the chart widget doesn't seem to handle the case well. And I'm guessing something changed between 4.1.0 and 4.1.1 in this area.
I found the problem. I was specifying a convert function for the chart store to a variable with an undefined value in some cases. It worked fine in 4.1.0 rc3, but in 4.1 release, it did not work. Even after I assigned the variable to a function, it still didn't work unless I coded the function to convert my string values to numbers explicitly. The store didn't seem to consistently auto-convert strings to numbers for Numeric series. So in short, it looks like the newer code requires the user to do the string to number conversions.