text: Title: Geopackage Views seem not to work, or is the DateTime column?
  Body: Not sure if I do something wrong or if I hit an issue:
  
  Trying to create a scatter/line plot from a view from a Geopackage (Time-Value) where Time is a DateTime do not show up in DataPlotly:
  
  ![Screenshot-20200508163103-1269x910](https://user-images.githubusercontent.com/731673/81415923-5735b880-9149-11ea-8ae9-5fae96003dec.png)
  
  While the underlying data-table does this fine:
  
  ![Screenshot-20200508162937-1265x906](https://user-images.githubusercontent.com/731673/81415744-22c1fc80-9149-11ea-8999-896a11b375ad.png)
  
  **To Reproduce**
  Steps to reproduce the behavior:
  1. From this zipped geopackage: [cloud3.zip](https://github.com/ghtmtt/DataPlotly/files/4599597/cloud3.zip) load both the data table and the view4
  2. View4 is actually a view/join from Data and Grid
  3. Try to create a scatterplot from Data: works fine: as you see Value are actually VERY small values (from 1E-10 till 10 or so...)
  4. Then try to do this with the view4 layer: not sure what goes wrong, but the Y-axis is going from 0-4 but nothing is shown
  5. Note that I actually wanted (as this is time related data) see a plot of one cell (this is air dispersion modelling).. Also @ghtmtt interest probably: https://github.com/qgis/QGIS/issues/36291 and https://github.com/qgis/QGIS/issues/26804 I created the view with 'OGC_FID' and then selection works \o/
  6. Note that is also seems that for the view the DateTime (Time) column seems not to be reckognized?
  
  **Desktop (please complete the following information):**
   - OS: Debian Testing
   - QGIS master
   - DataPlotly current
  