Use explorer as method

[ahuang11]

Closes #1153

import hvplot.pandas

from bokeh.sampledata.penguins import data as df

df.hvplot.explorer(title="ABCD", kind="scatter", x="flipper_length_mm", y="body_mass_g", by=["species"])

[maximlt]

hvPlot's main entry points are so far:

• df.hvplot.<ptype>()
• df.interactive()
• hvplot.explorer(df)

The first two approaches are patching the data structures that support their operations, dynamically adding the hvplot and interactive accessors. For some reason, this wasn't done with explorer in the first place. Maybe because the explorer is quite different in nature compared to the other two accessors, being basically a not-so-small app. There's also the possibility (not sure it works!) that you can create an explorer from some dataset and update it with another dataset, maybe from a different datatype entirely. That's actually a valid use case for the explorer, imagine a Lumen app filtering/transforming some data that ends up in a pre-defined explorer for more exploration. The more common use case though for the explorer is to operate on a single dataset. In that case, the API offered is inconsistent with the two other entry points and and this is what this PR aims to improve.

One could suggest that hvPlot could add an explorer accessor as it does with hvplot and interactive. I thought about that a little but prefer the approach of this PR, adding a new explorer method under df.hvplot, as the explorer is very close to the plotting API (a lot more than .interactive) and GeoPandas has recently added an .explore method (to get an interactive plot), which could be a little confusing for their users.

@ahuang11 there needs to be a few more docs update:

• landing page
• explorer getting started

And I have left a few comments.

[maximlt]

Also now the explorer has Xarray support I believe this can be moved to hvPlotBase

[maximlt]

Amending myself, it's not yet supported as #1141 hasn't been merged, but will be soon I guess so this PR could be adapted accordingly if not merged before.

[ahuang11]

Okay I suppose we can wait until that's done first.

[maximlt]

I have made these changes:

• moved the explorer method to hvPlotBase
• replaced explorer handling in converter.py by handling in hvPlotBase.__call__; I prefer not to have any explorer code in the HoloViewsConverter, it's complicated enough :D