[Paraview] Integrate spatially and temporally

[Alex Lindsay]

Hi all,

I have a data set for which I would like perform both spatial and 
temporal integration. I can easily do the spatial integration with 
`IntegrateVariables`. I've set up a `PlotSelectionOverTime` filter that 
allows me to plot the spatially averaged value over time. However, I 
don't know how to integrate this curve. If I try to derive directly from 
`PlotSelectionOverTime`, `IntegrateVariables` is grayed out. I've tried 
doing

1. `TableToPoints` filter using `Time` as one of the coordinates (derive 
from `PlotSelectionOverTime`)
2. Select all resulting "points" in either the `Render` or `Spreadsheet` 
views
3. `IntegrateVariables` (no longer grayed out)

but this yields `0` for all of my fields.

I ran across [this 
thread](http://marc.info/?l=paraview&m=128804494805006&w=2) from years 
ago but things aren't panning out for me.

I'm using Paraview 5.2.0.

Alex