Color selection improvements

[meguiraun]

• Show currently selected color next to 'pick color'
• An 'ok' button for closing the color picker panel
• Be able to change the color for already existing plots, be aware that the user may want to re-color several plots, so keep the changes onhold and apply them via a new button
• reset color selection when loading attribute list
• When the user does not select a color, display as well what will be the default displayed color

[johanfforsberg]

The main problem with changing the colors of the plots right now is that the plot needs to be redrawn each time anything changes. This is pretty stupid but a consequence of how the plot is generated using "datashader". All the lines are drawn into one image on the server using the configured color, and then the finished image is delivered to the client.

I've been thinking about a solution to this and I think it would be pretty simple and provides several benefits. Each line could be drawn to separate images instead of in the same image, using white color, and sent separately. Then it's up to the client to render the plots using any color onto the screen. This would use some more bandwidth (although not too much as the images would be monochromatic) but the main benefit would be that changing color on a plot would be instant. It would also be possible to quickly hide/show individual lines to make it easier to differentiate the lines.

I can have a look at implementing this if it makes sense!