Use an SVG gradient.
Add the gradient definition as a separate snippet of SVG in your HTML file:
<span class="chart">100, 100.67, 102.34, 109.30, 101.20, 99.20, 70.20</span>
<svg>
<defs>
<linearGradient id="grad">
<stop offset="0" stop-color="red"/>
<stop offset="1" stop-color="blue"/>
</linearGradient>
</defs>
</svg>
Then refer to it in your CSS:
polygon {
fill: url(#grad);
}
You can learn more about SVG gradients here: