Empty scatter plot in Altair

Question

I am very puzzled on why this code does not work:

#@title Visualize trends in accelerators

x_axis = 'Release Year'
y_axis = 'FP32 (single precision) Performance [FLOP/s]'

# Libraries
import pandas as pd
import numpy as np
import altair as alt

# Download data
key = '1AAIebjNsnJj_uKALHbXNfn3_YsT6sHXtCU0q7OIPuc4'
sheet_name = 'HARDWARE_DATA'
url = f'https://docs.google.com/spreadsheets/d/{key}/gviz/tq?tqx=out:csv&sheet={sheet_name}'
df = pd.read_csv(url)

# Filter NaN datapoints
df = df[~df[x_axis].isna()]
df = df[~df[y_axis].isna()]

# Plot the dataset
alt.themes.enable('fivethirtyeight')
alt.Chart(df).mark_circle(size=60).encode(
  x=alt.X(f'{x_axis}:Q',
          scale=alt.Scale(
                          domain=(df[x_axis].min(), df[x_axis].max())
                          ),
          axis=alt.Axis(format=".0f")
          ),
  y=alt.Y(f'{y_axis}:Q',
          scale=alt.Scale(
                          domain=(df[y_axis].min(), df[y_axis].max())
                          ),
          axis=alt.Axis(format=".1e")
          ),
)

It works when I plot it using seaborn

import seaborn as sns
sns.set_theme()
sns.regplot(x=df[x_axis], y=df[y_axis]);

No error message is shown - just the empty plot. The console throws this warning

DevTools failed to load source map: Could not load content for https://cdn.jsdelivr.net/npm/vega-embed.min.js.map: HTTP error: status code 404, net::ERR_HTTP_RESPONSE_CODE_FAILURE

What is going on?

user_2937831 · Answer

The issue is that you have special characters in your column name, which need to be escaped in Altair (see e.g. the field documentation in https://altair-viz.github.io/user_guide/generated/core/altair.Color.html?highlight=escape)

Why is this? Characters like . and [] in Vega-Lite are used to access nested attributes of columns.

The easiest approach would be to avoid such special characters in your dataframe column names. Alternatively, you can escape the special characters with a back-slash (\) though be careful about the effect of back-slashes in Python strings (use an r prefix for raw string encoding). For example:

x_axis = 'Release Year'
y_axis = 'FP32 (single precision) Performance [FLOP/s]'
y_axis_escaped = r'FP32 (single precision) Performance $$FLOP/s$$'

# Libraries
import pandas as pd
import numpy as np
import altair as alt

# Download data
key = '1AAIebjNsnJj_uKALHbXNfn3_YsT6sHXtCU0q7OIPuc4'
sheet_name = 'HARDWARE_DATA'
url = f'https://docs.google.com/spreadsheets/d/{key}/gviz/tq?tqx=out:csv&sheet={sheet_name}'
df = pd.read_csv(url)

# Filter NaN datapoints
df = df[~df[x_axis].isna()]
df = df[~df[y_axis].isna()]

# Plot the dataset
alt.themes.enable('fivethirtyeight')
alt.Chart(df).mark_circle(size=60).encode(
  x=alt.X(f'{x_axis}:Q',
          scale=alt.Scale(
                          domain=(df[x_axis].min(), df[x_axis].max())
                          ),
          axis=alt.Axis(format=".0f")
          ),
  y=alt.Y(f'{y_axis_escaped}:Q',
          scale=alt.Scale(
                          domain=(df[y_axis].min(), df[y_axis].max())
                          ),
          axis=alt.Axis(format=".1e")
          ),
)

Empty scatter plot in Altair

1 Answers