Hi @nodedge,

Thanks for the PR. I'm in support of the feature, but in its current implementation it breaks default behavior.

For example, running python example/GraphicsScene.py on this PR.

Same thing but on master right now:

This can likely be worked around by checking if the pen has a default width, in which case set the pen width to self.pixelWidth() but otherwise set to the width you're working with.

Or alternatively a "better" implementation IMO would be to multiply the pen width by self.pixelWidth(); as it makes sense to have the pen width of the GridItem be in screen pixels, and have nothing to do with the coordinate system it's in.

@NilsNemitz @ixjlyons do either of you have input.

Probably the simplest way to handle this is to just extract the width in setPen and use that in generatePicture. Something like:

diff --git a/pyqtgraph/graphicsItems/GridItem.py b/pyqtgraph/graphicsItems/GridItem.py
index efb1afdb..ff9f8b6f 100644
--- a/pyqtgraph/graphicsItems/GridItem.py
+++ b/pyqtgraph/graphicsItems/GridItem.py
@@ -35,6 +35,7 @@ class GridItem(UIGraphicsItem):
             self.opts['pen'] = fn.mkPen(*args, **kwargs)
 
         self.picture = None
+        self._penWidth = self.opts['pen'].width()
         self.update()
 
 
@@ -157,7 +158,6 @@ class GridItem(UIGraphicsItem):
 
                 linePen = self.opts['pen']
                 lineColor = self.opts['pen'].color()
-                lineWidth = self.opts['pen'].width()
                 lineColor.setAlpha(c)
                 linePen.setColor(lineColor)
 
@@ -171,9 +171,9 @@ class GridItem(UIGraphicsItem):
                 for x in range(0, int(nl[ax])):
                     linePen.setCosmetic(False)
                     if ax == 0:
-                        linePen.setWidthF(lineWidth)
+                        linePen.setWidthF(self._penWidth*self.pixelWidth())
                     else:
-                        linePen.setWidthF(lineWidth)
+                        linePen.setWidthF(self._penWidth*self.pixelHeight())
                     p.setPen(linePen)
                     p1 = np.array([0.,0.])
                     p2 = np.array([0.,0.])

@nodedge , @j9ac9k
Without having the time to check in detail:
It might also be useful to make sure that the QPen.setCosmetic() gets set?
see: https://doc.qt.io/qt-5/qpen.html#isCosmetic
I assume that the line width is supposed to be constant, and not a function of the zoom-level?

Ahh yes, GridItem pens should most certainly be cosmetic.

It's explicitly set to False - not sure why.

Thank you for your answers.
@ixjlyons I have just tried your solution.

The example "GraphicsScene" is not broken anymore.
I added a custom pen in this same example, but the gridline width does not seem to be modified.
Instead, there is more space between the lines.
If we maximize the window, the grid spacing seems to be reset.

I think I got to the bottom of this issue. The problem is this line I believe:

https://github.com/pyqtgraph/pyqtgraph/blob/master/pyqtgraph/graphicsItems/GridItem.py#L152

Since QPen's are mutable, we change the width of the pen's further down here:

https://github.com/pyqtgraph/pyqtgraph/blob/master/pyqtgraph/graphicsItems/GridItem.py#L166-L169

That change in width sort of throws out the initial setting we made.

We can work around this by modifying

                linePen = fn.mkPen(self.opts['pen'])

This returns a copy of the pen, where the width that was initially set to is preserved.

@nodedge would you be ok with me pushing my changes to your branch?

I think this can be closed now, see #2304

Thanks for your efforts @nodedge