add new "proportional" tick mode

What

Add a new tick mode:

# Usage:
xaxis: {
    tickmode: 'proportional',
    tickvals: [0, .5, 1],
}

Effect:

Plotly will make ticks propotional to the range: So if the x range is [50, 150], the tick marks will appear at 50, 100, and 150. If the xrange is [-0.5, 1.5], the tick marks will appear at -0.5, 0.5, and 1.5.

Why

Auto tick formatting is usually ugly (for any value ntick), and the other modes aren't responsive to user zooms.

Workaround: Using python w/ jlab, but I can calculate ticks manually on range changes and during rendering, and then use array mode, but it is clunky, and requires my users to have the plotly installed in their kernel and on jlab server. My proposal doesn't seem that different than modifying auto behavior, so I'll take a shot.

Who am I

Working on analysis software for geologists who design wells.

No source file or test is named in the issue. Start by locating the existing tick-mode handling and its tests, then compare the requested proportional examples with current auto and array behavior. Done means the new mode responds to range changes and produces ticks at the requested proportional values.