I'm using a line graph to show trends in a multi-question
questionnaire. Like many else (I've done a lot of searching but haven't
found a suitable solution) my line graph jumps to zero for the empty
cells (currently defined as ""). I understand I could just change this
to na() instead but then my average() calculation stops working. I have
a lot of data so making two separate data-series is not feasible.

Can anyone help me with a suggestion here? Thanks!