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
this._transformStack.push(old);
      this._transformMatrix = old.slice(0, 6);
      this._originalSave();
    };
    ctx.restore = function ctxRestore() {
      const prev = this._transformStack.pop();
      if (prev) {
        this._transformMatrix = prev;
        this._originalRestore();
      }
    };
    ctx.translate = function ctxTranslate(x, y) {
      const m = this._transformMatrix;
      m[4] = m[0] * x + m[2] * y + m[4];
      m[5] = m[1] * x + m[3] * y + m[5];
      this._originalTranslate(x, y);
    };
    ctx.scale = function ctxScale(x, y) {
      const m = this._transformMatrix;
      m[0] = m[0] * x;
      m[1] = m[1] * x;
      m[2] = m[2] * y;
      m[3] = m[3] * y;
      this._originalScale(x, y);
    };
    ctx.transform = function ctxTransform(a, b, c, d, e, f) {
      const m = this._transformMatrix;
      this._transformMatrix = [
        m[0] * a + m[2] * b,
        m[1] * a + m[3] * b,