Replace Transform

[domoritz]

I want to be able to specify what text is shown in the legend for ordinal colors but setting the domain and range only let's me override the colors but not the labels.

See vega/altair#278

[kanitw]

Users should simply derive a new data field and use the new field.

That said, I see that this would be painful:

transform: {
    calculate: { 
       expr: "datum.name === 'tom' ? 'Thomas' : datum.name === 'dick' ? ... : ... "
       as: "fullname",  // `field:` in v1
    }
}

Instead of making this specific to legend, I propose that we add a replace transform that can be useful for all of axis label, legend label, text marks, and the future label marks.

transform:  {
   replace: {
      field: 'name',
      as: 'fullname',
      dict: { 'tom' : 'Thomas', 'dick' : 'Richard', ... }
   }
}

[kanitw]

A syntax that might be more appropriate for the new transform array might be:

transform: [{
   replace: 'name'
   to:  { 'tom' : 'Thomas', 'dick' : 'Richard', ... }
   as: 'fullname'
}]

[kanitw]

The code that @sirahd add to generate sort index in #3423 is definitely useful for this "replace" transform.

[domoritz]

Yes. Close this then?

[kanitw]

? Having reuseable code for this task doesn't mean we support this task by any mean.

[domoritz]

Oh, I meant close this issue and don't support replace.

[kanitw]

Why? The current way to do replace is still super annoying. (Write ugly chains of ternary operator)

[domoritz]

Never mind. I misunderstood that this issue has some other benefits besides sorting domains.

[kanitw]

this issue has some other benefits besides sorting domains.

Yes it does ;)

[domoritz]

Cool. Somebody implement this then :-P

[kanitw]

I wonder if it would be best implemented as a function in Vega expression though so it will benefit the whole stack.

[domoritz]

I defer that question to @jheer

[kanitw]

Actually something like {'Sat': 'Weekend', 'Sun': 'Weekend'}[day] || 'Weekday' would be sufficient for this purpose.

[domoritz]

We use this in #4020 😛

[domoritz]

It might be worth documenting this somewhere.

[jowens]

I'm at a loss as to how this was actually fixed / how this can be expressed ... is @kanitw's sat/sun/weekend example how this can be expressed now?

[domoritz]

Yes, you can write a calculate transform with an expression as in @kanitw's comment.