Altair Scatter Plot tickMinStep

Question

I have this DataFrame:

    x           y           term        s
0   0.000000    0.132653    matlab      0.893072
1   0.000000    0.142857    matrix      0.905120
2   0.012346    0.153061    laboratory  0.902610
3   0.987654    0.989796    be          0.857932
4   0.938272    0.959184    a           0.861948

And have generated this chart:

Using this code:

chart = alt.Chart(scatterdata_df).mark_circle().encode(
        x = alt.X('x:Q', axis = alt.Axis(title = "⏪  less physical | more physical ⏩", tickMinStep = 0.05)),
        y = alt.Y('y:Q', axis = alt.Axis(title = "⏪  less data | more data ⏩", tickMinStep = 0.05)),
        color = alt.Color('s:Q', scale=alt.Scale(scheme='redblue')),
        tooltip = ['term']
    ).properties(
        width = 500,
        height = 500
    )

Having set tickMinStep = 0.05 for both the x and y axes, I'm not sure why the graph doesn't reflect this parameter.

Thank you.

Answer 1

tickMinStep specifies the minimum tick spacing for the automatically determined ticks; actual tick spacing may be larger than this. It sounds like you want more ticks than the default heuristic gives, so you should instead adjust tickCount directly:

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

rng = np.random.default_rng(0)

data = pd.DataFrame({
    'x': rng.random(size=100),
    'y': rng.random(size=100)
})

alt.Chart(data).mark_point().encode(
    x=alt.X('x:Q', axis=alt.Axis(tickCount=20)),
    y=alt.Y('y:Q', axis=alt.Axis(tickCount=20)),
)

If you want even finer control over tick values, you can pass a list of values directly to Axis.values. For more information, see the alt.Axis documentation.