I would think your calculation would work for this if it produces an empty result in all cases except those that fall on the 30 minute interval. You can specify such a calculation in the chart setup. Haven't tried this though so I can't be positive that it will work.
More sophisticated means for controlling axis values and labels in FileMaker Charts would make a very welcome future improvement...
For those that agree, you can use the Feature Suggestion Form to so inform FileMaker Inc.
As it turns out, YES the calculation works fine. When I wrote it, I stopped before even trying it, because I said to myself, "Wait, this won't work, it'll only plot points at the half-hour marks." What I didn't realize is that what I thought was the "X-axis value" is NOT a value, it's the LABEL. The value used for the X axis is the record number in the found set! I had thought that irregular time periods would be plotted appropriately based on the "X-axis value", say, if the data only had 5-second-spaced samples for part of the time, and 1-second-space samples for another part, that the 5ers would be plotted further apart from each other. But that's not the case. Each record is a vertical column of equal width, with the Y axis value plotted in it, not an X-Y scatter of points based on 2 values.
So, the method does work. I've attached a second stab at the chart which shows the result, but as a demonstration of the shortcoming, I made 60 duplicates of the 19:30 data point to illustrate that that data comes at the end (where duplicates go, record-wise, unless sorted) rather than stacking points up at the presumed 19:30 spot on the chart. The smear at the end is a bunch of 19:30 texts stepping on each other. Also, the overshoot is because I was using curve-smooth, which, considering the data, isn't the right choice since it's pretty smooth anyway.
So, to really make this work the way I want, I think I need to create a table of time periods, say each one with a start and end time five seconds apart and sequential, and make a relation to the logged data to get and average any logged data in that period. That way, even if the data is jagged in the logging period, the spacing of the display points will be time-consistent.
I think that you'll need to interpolate data points for records where the time interval does not include a value in order to keep Filemaker from charting a zero value for that record.
Phil, you are right. Here's the same chart with data holes in it. The missing values are blank, not zero, but are interpreted as zero I guess. This is with poly segments selected rather than curves, which looks even worse.
Do you know how to interpolate the data? (Ironically, I first ran into this in an Excel chart years ago and used VBA with MS Access to interpolate the missing points to get the proper curve in the chart.)
Basically, you can use a looping script and the point-slope formula for graphing a linear relationship between two points to fill in the needed missing data points.