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.

Nell’issue non sono indicati né un file sorgente né un test. Inizia individuando la gestione esistente di tick-mode e i relativi test, quindi confronta gli esempi proporzionali richiesti con il comportamento attuale di auto e array. Il lavoro è completo quando la nuova modalità reagisce alle modifiche dell’intervallo e produce tick ai valori proporzionali richiesti.