Add support for "filled" graphs in the graph component

**Describe the problem you have/What new integration you would like**

Add an option to the [graph component](https://esphome.io/components/display/index.html#graph-component) to allow drawing a "filled" graph i.e. where the whole graph area is filled with a colour, rather than drawing just a line.

**Please describe your use case for this integration and alternatives you've tried:**

For some kind of displays/visualisations it's much more impactful/readable from a distance to show a filled chart. See example below of something I've implemented manually to draw on an epaper display.

**Additional context**

Obviously this is only easy to implement when showing a single trace. With multiple traces the layering might be hard to figure out. But I guess in an initial version this option can be restricted to graphs containing only one trace, by validating the YAML input?