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
get: function getCurrentTransform() {
        return this._transformMatrix;
      },
    });
    Object.defineProperty(ctx, "mozCurrentTransformInverse", {
      get: function getCurrentTransformInverse() {
        
        
        
        const m = this._transformMatrix;
        const a = m[0],
          b = m[1],
          c = m[2],
          d = m[3],
          e = m[4],
          f = m[5];
        const ad_bc = a * d - b * c;
        const bc_ad = b * c - a * d;
        return [
          d / ad_bc,
          b / bc_ad,
          c / bc_ad,
          a / ad_bc,
          (d * e - c * f) / bc_ad,
          (b * e - a * f) / ad_bc,
        ];
      },
    });
    ctx.save = function ctxSave() {
      const old = this._transformMatrix;