fix: Remove the arrow up and down from the column name in the dataframe

[glemaitre]

When showing a score dataframe, e.g. report.metrics.report_metrics(), the column name contains some arrow up/down depending if we show a score or a loss. However, adding this information directly in the column is annoying to index a specific score.

We should come with another way to provide this information. Maybe, a dirty way but possible way would be to patch the HTML repr from pandas for those specific dataset to incorporate the information in the HTML representation itself.

[glemaitre]

From a discussion with @adrinjalali IRL, we could make the metric being the row instead of column of the dataframe. It allows 2 things:

• we can add a new column with the "favorability" (I think it is called like this in some part of the code) that show the indicator for each columns and it could be controlled by a parameter if someone wants the info.
• for this to works it means that we need to transpose the report.

Transposing makes sense because right now, if one want to plot with pandas, you need to write:

report.metrics.report_metrics().T.plot.barh()

We can simplify it to

report.metrics.report_metrics().plot.barh()

which make total sense to me.

[glemaitre]

An additional thought is about polars/pandas dataframe. Currently, we have multindex by default (because I like them). polars would support it, @GaelVaroquaux and @adrinjalali complain about them.

I think that we all agree that multindex are nice when we look at the HTML representation but they are not easy to deal with when indexing. Therefore, I think that we need a parameter to explicitely request flat vs. multindex. For instance, in the model cards, we want only the representation and multindex make sense. When a user call report.metrics.report_metrics, we can provide one an option to take the type of index that one knows how to deal with.

[sylvaincom]

Thanks, +1 for "multi index are nice for visualization but not easy for indexing"

[glemaitre]

I also think that it should be in the 0.7 milestone. Once we transpose, it will be pretty straightforward. ping @MarieS-WiMLDS

[MarieSacksick]

Same as the other issue: if it's ready, it's a bonus! It will be at least necessary for 0.8 where we want to include improvement about the reports.
I'm adding it to the next milestone to show that it's not a blocker for 0.7 release, but it doesn't mean it's forbidden to start it before hand obviously :)!