you may have a look at the mapping functions, there you could also define a default value.
Without trying myself, I think something like
should work also.
Alternatively, you might change the NULL value representation symbol (which could be also a text) .e.g. in chart properties presentation tab.
Thanks for your help!
Before seeing your post I have already tried below code which you have suggested and looks like it works but still not tested though.But felt happy that same thing has been suggested by you. :-)
Yet to look into the mapping function which you have suggested. But don't think changing null value representation will work out as I need to use it for calculation in the expression.
Will try to look into the mapping function and will get back to you.