Groups and stacked/dodged positions.

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技术栈
numpy, pandas, python
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data-visualization

调研方向

首先使用提供的 full_df 数据复现 geom_col、position='stack'、position='dodge' 和 position_dodge2 的示例。跟踪这些 position 如何处理显式的 group='replicate' 和 fill='species' 映射；完成后应提供一种有文档记录且经过测试的方法，在保持 species 堆叠的同时对 replicate 组进行 dodge，而无需手动调整 x 或 width。

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描述

Discussion Feature

I'm dealing with a figure where I would like to be able to dodge columns based on the group aesthetic while having the columns stay stacked on the fill aesthetic.

My intuition was that for geometries that use the group aesthetic, color/fill is assumed to be used as the group when it is not explicitly mapped to a variable. This seems to be the case with geom_line, for instance.

However, when using position_dodge and an explicitly mapped group, the bars appear to still be dodged by the group + fill variable.

To show some examples with a toy dataset:

df = pd.DataFrame(dict(
    day=np.repeat(range(1, 5), 3),
    species=np.tile(['A', 'B', 'C'], 4),
    counts=np.random.randint(0, 200, 12),
    replicate=1
))

df_replicates = df.query('day==2').assign(replicate=2).assign(counts=np.random.randint(0, 200, 3))

full_df = pd.concat([df, df_replicates])
day species counts replicate
1 A 49 1
1 B 92 1
1 C 195 1
2 A 110 1
2 B 150 1
2 C 32 1
3 A 9 1
3 B 146 1
3 C 195 1
4 A 161 1
4 B 190 1
4 C 100 1
2 A 88 2
2 B 87 2
2 C 32 2

In this case, we have two different samples/replicates in day 2.

Default Behaviour

(p9.ggplot(full_df)
 + p9.aes('day', 'counts', fill='species', group='replicate')
 + p9.geom_col()
)

image

Since the default position is stack, specifying the group as the replicate stacks the bars for each of the replicates of day 2 following the order of species-replicate: A-1, B-1, C-1, A-2, B-2, C-2. Fair enough, this is probably the expected and desired behaviour.

Dodged Behaviour

The issue arises when we attempt to dodge the groups, so that the height of the bars still represents the total counts in a single sample:

position_dodge
(p9.ggplot(full_df)
 + p9.aes('day', 'counts', fill='species', group='replicate')
 + p9.geom_col(position='dodge')
)

image

The group (replicates) have now been properly dodged/unstacked in day 2, but the previously stacked species all start now in the same x-y coords of the day-group instead of being stacked. Lowering the alpha a tad helps to see it better:

image

position_dodge2

The results with position_dodge2 are also problematic, since the width of the bars doesn't seem to be properly calculated (day 1 values become half as wide, while values for day 2 stay the same and overlap with values from the other days).

In any case, this completely unstacks the species/fill aesthetic, which is something we don't want even if bars' width was properly calculated.

The hacky solution

I thought that manually changing the x values (works in this case but would be a bit harder if the x variable was a Categorical, for instance) and generating a width variable to pass as an aesthetic manually setting the width of the day 2 values to .25 should allow me to make this work:

(full_df
 .assign(day=lambda dd: np.where(dd.replicate == 2, dd.day + .22, dd.day))
 .assign(day=lambda dd: np.where(dd.day==2, dd.day - .22, dd.day))
 .assign(width=lambda dd: np.where(dd.day.round() == 2, .4, .8))
 .pipe(lambda dd: p9.ggplot(dd)
 + p9.aes('day', 'counts', fill='species', group='replicate')
 + p9.geom_col(position='stack', mapping=p9.aes(width='width'))
)

image

I actually managed to solve this while writing the issue with this hacky solution, but still, it feels like there might be some way to include this behaviour in some position variety? I don't know if this situation is common enough to justify generating a specific position_dodgestack or something like that, but what do you think?

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