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.

Issue 中没有指定源文件或测试。首先定位现有的 tick-mode 处理逻辑及其测试，然后将请求的比例示例与当前 auto 和 array 的行为进行比较。当新模式能够响应范围变化，并在请求的比例值处生成 ticks 时，即表示完成。