Tableau doesn't currently let you turn-off linear interpolation when displaying line and area charts. As you've found out, that can lead to misleading visualizations if there are gaps in time between data values (assuming time is the x-axis). If you'd like to be able to choose between step plots and linear interpolation, upvote the following idea http://community.tableau.com/ideas/1067
The best workaround I've found is to fill in the data between measurements in effect repeating each measurement at later times until just before the next change in value. You can do that by a little custom sql or pre-processing the data automatically or in the worst case by hand.
I gave an example of how to do this by custom (Oracle) sql in the tips in the wild session at the 2012 Tableau customer conference. If you can find the video and workbook online, that might help if your example was similar. The basic idea was to write SQL to generate a table of zeros for each date/time in range, then union those values with the original data, and finally plot a running sum. ( I can't post it at the moment, but will look for it later on)
Thanks for your response Alex.
I actually want the linear interpolation to occur. The problem is that when I filter on one sheet and it affects another (in a dashboard), and the drill down occurs between points (or there is only one point), the linear interpolation doesn't occur.
This example is what happens when I filter and there is only one point. I want the light blue line to be continuous, like the dark blue line.
Let me know if I can clarify further. Thanks!
This is an example of what I'd like to happen when I drill down down (i.e filter from one viz to another).
This is what happens currently:
So, because I only technically have one point on the drill down, it is not drawing a line between the points outside of the second viz, which I would like it to do.