Changing Number of y-axis Ticks in Altair

I've generated this image comparing Bitcoin to REIT yields over the ten-year period from 2010-2020 using a symmetric log y-axis:

The plot was generated from this DataFrame:

    btc_yields  Year    reit_yields
0   3619143193  2010    38791
1   45790476    2011    31261
2   37433124    2012    30274
3   2739779     2013    4051
4   808407      2014    282
5   1711472     2015    650
6   685726      2016    863
7   75958       2017    1324
8   49428       2018    976
9   34330       2019    -285
10  27005       2020    631

Using this code:

alt.Chart(merged_btc_reit_low_inf).transform_fold(
    ['btc_yields', 'reit_yields'],
).mark_line(strokeWidth = 5).encode(
    x=alt.X('Year:N', title = 'Year Investment Made'),
    y=alt.Y('value:Q', title = 'Returns in USD', 
            scale=alt.Scale(type='symlog'),
            axis=alt.Axis(tickCount=merged_btc_reit_low_inf.shape[0])),
    color='key:N',
    tooltip = 'value:Q'
).properties(
    width = 600,
    height = 400,
    title = 'Return on $10,000 Seed Investment in Bitcoin'
).configure_axis(
    labelFontSize=14,
    titleFontSize=16,
    labelAngle = 0
).configure_title(
    fontSize = 21
).configure_legend(
    labelFontSize = 14
)

I am not sure why, even with axis=alt.Axis(tickCount=merged_btc_reit_low_inf.shape[0]) there are only 3 y-axis ticks. I would like a hand increasing the number of y-axis ticks to enough that the plot is more informative, especially considering the log-transformed axis.

You could set the axis values explicitly instead of trying to adjust the number of ticks:

from itertools import product

...

axis=alt.Axis(values=[0] + [10**x * y for (x, y) in product(range(2, 9, 2), (1, -1))]))
# There might be a nicer way of composing this list...

enter image description here

I think the reason tickCount is not working for you is this section from the docs:

The resulting number may be different so that values are “nice” (multiples of 2, 5, 10) and lie within the underlying scale’s range.

I am not sure why they are not symmetrical, this parameter might not work well with symlog so you might want to open a feature request for that at the VegaLite repo.

Changing Number of y-axis Ticks in Altair

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