Graph: fixed png rendering with legend to the right

[torkelo]

Upgraded autoprefixer changed the way flexbox prefixing is done

Before with autoprefixer 6.4.0

.graph-panel {
  display: -webkit-box;
  display: -ms-flexbox;
  display: flex;
  -webkit-box-orient: vertical;
  -webkit-box-direction: normal;
      -ms-flex-direction: column;
          flex-direction: column;
  height: 100%; }

With autoprefixer 9

.graph-panel {
  display: -webkit-box;
  display: -webkit-flex;
  display: -ms-flexbox;
  display: flex;
  -webkit-box-orient: vertical;
  -webkit-box-direction: normal;
  -webkit-flex-direction: column;
      -ms-flex-direction: column;
          flex-direction: column;
  height: 100%; }

Major difference is the new prefix display: -webkit-flex; after the display: -webkit-box;

This addition of the newer prefix standard caused the legend to the right to stop working.

Fixes #16443