Steps to reproduce the problem:
1. When using the demo application - https://mozilla.github.io/pdf.js/web/viewer.html, load  the document 5311693-1.pdf

[5311693-1.pdf](https://github.com/mozilla/pdf.js/files/5137225/5311693-1.pdf)

2. go to Page 16 of the document.

3.  The strip of  EKG data  appears to have an incorrect line width   under the  Multi-color heat rate chart.

4.  When the same document is generated by pdffactory ,  the artifact are not visible.

What is the expected behavior? (add screenshot)
![expected](https://user-images.githubusercontent.com/3577617/91461157-4622cb80-e84e-11ea-8e5d-750c7b4e64ae.PNG)

What went wrong? (add screenshot)
The EKG line strip  has an incorrect thickness.  the x-transform seems to stretch the line thickness.  See the  stri in the RED rectagngle of the attached behavior versus the GREEN rectangle

Link to a viewer (if hosted on a site other than mozilla.github.io/pdf.js or as Firefox/Chrome extension):
 https://mozilla.github.io/pdf.js/web/viewer.html
warn("Knockout groups not supported.");
      }
      const currentTransform = currentCtx.mozCurrentTransform;
      if (group.matrix) {
        currentCtx.transform.apply(currentCtx, group.matrix);
      }
      if (!group.bbox) {
        throw new Error("Bounding box is required.");
      }
      
      
      let bounds = Util.getAxialAlignedBoundingBox(
        group.bbox,
        currentCtx.mozCurrentTransform
      );
      
      const canvasBounds = [
        0,
        0,
        currentCtx.canvas.width,
        currentCtx.canvas.height,
      ];
      bounds = Util.intersect(bounds, canvasBounds) || [0, 0, 0, 0];
      
      
      const offsetX = Math.floor(bounds[0]);
      const offsetY = Math.floor(bounds[1]);
      let drawnWidth = Math.max(Math.ceil(bounds[2]) - offsetX, 1);
      let drawnHeight = Math.max(Math.ceil(bounds[3]) - offsetY, 1);
      let scaleX = 1,