Scatter Plot's show density does not work with pyqtgraph 0.10

[astaric]

Update pyqtgraph to the latest version, then connect iris to scatter plot and select "Show class density". The background changes to gray, while it should change to a colored map of class densities.

If pyqtgraph is downgraded to 0.9.*, class density works.

[ajdapretnar]

@astaric Seems like this has been fixed?

[astaric]

Fixed in #1990