AFAIK, That is not chart.js adding a background color to your component but rather your browser displaying that you have this area under selection. I don't think this can be resolved unless you disable selection of the chart by adding this CSS to it.

.noselect {
  -webkit-touch-callout: none; /* iOS Safari */
    -webkit-user-select: none; /* Safari */
     -khtml-user-select: none; /* Konqueror HTML */
       -moz-user-select: none; /* Firefox */
        -ms-user-select: none; /* Internet Explorer/Edge */
            user-select: none; /* Non-prefixed version, currently
                                  supported by Chrome and Opera */

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