Format labels on bar charts in Altair - altair

Question

[![chart showing numbers without correct formatting][1]][1]
I need to format the label on these bars, so that they are rounded to nearest whole number. I have the following code:
def chart_tender_response_times(dataframe=None):
chart = (
alt.Chart(dataframe, title="Median time to respond to a tender")
.mark_bar()
.encode(
alt.X("year(date):O"
),
alt.Y("mean(median_duration):Q",
## This is our units section, only describe the units of measurement here.
axis=alt.Axis(title="Unit: days.")
),
alt.Tooltip(["mean(median_duration):Q"], format=",.2r", title="Days to respond to a tender")
)
)
text = (
chart.mark_text(align="center", baseline="bottom")
.encode(text='mean(median_duration):Q')
)
return chart+text
I've tried variations of the following...
text = (
chart.mark_text(align="center", baseline="bottom")
.encode(text='mean(median_duration):Q', format='.,2r')
)
but this returns the following schema validation error:
SchemaValidationError: Invalid specification
altair.vegalite.v3.api.Chart, validating 'required'
'data' is a required property
My hunch is that I have to somehow call and format the value, before adding it to the chart, but I can't see how to do this from either the documentation or the examples.

Answer 1

You need to wrap the format in alt.Text, as in encode(text=alt.Text('mean(median_duration):Q', format=',.2r'))
Also, I think format=',.0f' is more robust to round to the nearest integer (e.g. if you have 256.4, it would be rounded to 256, whereas with format=',.2r' you'd get 260)
Below is an example with a function a bit modified to fit another dataset (as you did not provide one):
import altair as alt
from vega_datasets import data
cars = data("cars")
def chart_tender_response_times(dataframe=None):
chart = (
alt.Chart(dataframe, title="Median time to respond to a tender")
.mark_bar()
.encode(
alt.X("year(Year):O"),
alt.Y(
"mean(Displacement):Q",
## This is our units section, only describe the units of measurement here.
axis=alt.Axis(title="Unit: days."),
),
alt.Tooltip(
["mean(Displacement):Q"],
format=",.0f",
title="Days to respond to a tender",
),
)
)
text = chart.mark_text(align="center", baseline="bottom").encode(
text=alt.Text("mean(Displacement):Q", format=",.0f")
)
return chart + text
chart_tender_response_times(cars)