the href encoding

Hi Jake and all,

Is the href encoding supported in Altair? I can’t tell by the docs how I would use it. Vega lite docs uses “as” in the spec and of course that is a key work in Python:

{
  "data": {"url": "data/cars.json"},
  "mark": "point",
  "transform": [{
    "calculate": "'https://www.google.com/search?q=' + datum.Name", "as": "url"
  }],
  "encoding": {
    "x": {"field": "Horsepower", "type": "quantitative"},
    "y": {"field": "Miles_per_Gallon", "type": "quantitative"},
    "color": {"field": "Origin", "type": "nominal"},
    "tooltip": {"field": "Name", "type": "nominal"},
    "href": {"field": "url", "type": "nominal"}
  }
}

[jakevdp]

Yes, it is supported. For example, here we modify a scatter plot of car properties so that clicking on any point opens a google search for the car name:

import altair as alt
from vega_datasets import data

try:
    # Python 3
    from urllib.parse import urlencode
except ImportError:
    # Python 2
    from urllib import urlencode

def make_google_query(name):
    return "https://www.google.com/search?" + urlencode({'q': '"{0}"'.format(name)})

cars['url'] = cars['Name'].apply(make_google_query)

alt.Chart(cars).mark_circle(size=60).encode(
    x='Horsepower',
    y='Miles_per_Gallon',
    color='Origin',
    href='url',
    tooltip=['Name', 'url']
)

This would probably be worth adding as an example in the docs

[dchudz]

Ah, I see if I set the masked rows to an empty string, it’s effectively what I want (the hyperlink seems active in the browser, but it’s a non-op when you click it.)

In case this helps anyone else:

I didn’t feel that was good enough for my use case – I really want there to be no active hyperlink when there is no href.

In my case it seems to work well to make two charts, and add them to together:

        with_urls = (altair.Chart(df[df.Url.notnull()])
        .encode(
            href="Url",
...
        ))

        without_urls = (altair.Chart(df[df.Url.isnull()])
        .encode(
            x="Date:O",
            y=altair.Y("test:N", title=None),
...   # no href
        ))

with_urls + without_urls