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
transform,
      viewport,
      transparency = false,
      background = null,
    }) {
      
      
      
      
      
      const width = this.ctx.canvas.width;
      const height = this.ctx.canvas.height;
      this.ctx.save();
      this.ctx.fillStyle = background || "rgb(255, 255, 255)";
      this.ctx.fillRect(0, 0, width, height);
      this.ctx.restore();
      if (transparency) {
        const transparentCanvas = this.cachedCanvases.getCanvas(
          "transparent",
          width,
          height,
          true
        );
        this.compositeCtx = this.ctx;
        this.transparentCanvas = transparentCanvas.canvas;
        this.ctx = transparentCanvas.context;