Realtime x/y coordinate tooltip

[netil]

Description

Provide option to display current pointer's x/y coordinate value.

• Option1: display tooltip on x/y axes area
  
• Option 2: display tooltip on x axis area
  

interface?

axis: {
     [x | y | y2] : {
          tooltip: {
               show: boolean;
               format(s: string);  // if not set, will follow tick.format
               grid: boolean; // whereas to show focused grid lines
           }
     },
}

• Considerations:
  • grid lines need to be displayed only one per axes.
    • for y/y2, only one horizontal line
  • how to handle grid line collision?
    • focus grid line will be displayed by default
    • also, grid.focus option will