enable display of different ticktext for hover and axis when using `hovermode='x unified'`

Hi,

Below you can read a feature request (given that this does not exist already).

The scenario

I am looking for a nice solution for displaying a hover "title" different from the ticks when using hovermode='x unified'.
Let's say that you want to display multiple time series and you have your range defined in 2 different ways. You have the timestamps, and the elapsed time. Now, you don't want to display both on the x axis to keep that clutter free, but you want to display them when hovering over the plot.

The problem

Take this code for example:

import plotly.graph_objects as go

x = ['2023-07-01', '2023-07-02', '2023-07-03', '2023-07-04']
y1 = [10, 15, 7, 12]
y2 = [5, 8, 10, 6]

ticktext = ['Day 1', 'Day 2', 'Day 3', 'Day 4']
tickvals = x  

hovertext = ['Hover 1', 'Hover 2', 'Hover 3', 'Hover 4']

fig = go.Figure()

fig.add_trace(go.Scatter(x=x, y=y1, hovertext=hovertext, name='Series 1'))
fig.add_trace(go.Scatter(x=x, y=y2, name='Series 2'))

fig.update_layout(xaxis=dict(
    tickmode='array',
    ticktext=ticktext,
    tickvals=tickvals
), 
    hovermode='x unified'
)

fig.show()

This will produce the following plot, and the problem is that I want to place "Hover 2" in place of "Day 2", while keeping "Day 2" on the x axis:

One solution

One solution I came up with was introducing a 2nd y axis, and placing the hover information there, hiding the axis and hiding the line. This works fine, however results in a hover display that looks "funny", essentially leaving some padding before the displayed text.

import plotly.graph_objects as go

x = ['2023-07-01', '2023-07-02', '2023-07-03', '2023-07-04']
y1 = [10, 15, 7, 12]
y2 = [5, 8, 10, 6]

ticktext = ['Day 1', 'Day 2', 'Day 3', 'Day 4']
tickvals = x  

hovertext = ['Hover 1', 'Hover 2', 'Hover 3', 'Hover 4']

fig = go.Figure()

fig.add_trace(go.Scatter(x=x,
                          y=y1,
                          text=hovertext,
                          yaxis='y2',
                          opacity=0,
                          showlegend=False,
                          hovertemplate = "%{text}<extra></extra>") )
fig.add_trace(go.Scatter(x=x, y=y1, hovertext=hovertext, name='Series 1', hovertemplate = "%{y:.1f}<extra></extra>"))
fig.add_trace(go.Scatter(x=x, y=y2, name='Series 2', hovertemplate = "%{y:.1f}<extra></extra>"))

fig.update_layout(xaxis=dict(
    tickmode='array',
    ticktext=ticktext,
    tickvals=tickvals
    ), 
    hovermode='x unified',
    yaxis2 = dict(
        overlaying='y',
        showticklabels=False
    ),
)

fig.show()

Proposed solution

It would be so amazing if one could just have a hoverticktext arg in the layout, that would allow for another piece of text to be set.

Imaginary example code with expected result:

import plotly.graph_objects as go

x = ['2023-07-01', '2023-07-02', '2023-07-03', '2023-07-04']
y1 = [10, 15, 7, 12]
y2 = [5, 8, 10, 6]

ticktext = ['Day 1', 'Day 2', 'Day 3', 'Day 4']
tickvals = x  

hovertext = ['Hover 1', 'Hover 2', 'Hover 3', 'Hover 4']

fig = go.Figure()

fig.add_trace(go.Scatter(x=x, y=y1, name='Series 1'))
fig.add_trace(go.Scatter(x=x, y=y2, name='Series 2'))

fig.update_layout(xaxis=dict(
    tickmode='array',
    ticktext=ticktext,
    tickvals=tickvals,
    hoverticktext = hovertext # added this imaginary line
), 
    hovermode='x unified'
)

fig.show()

And this would result in the following (edited picture) plot:

Start by reviewing the issue's Python examples and the existing axis ticktext, tickvals, hovertext, and unified-hover behavior in plotly.py. No source files or tests are named, so identify the relevant axis and hover entry points before deciding whether the change belongs in plotly.py or its underlying plotting layer. Done means separate hover-axis text works with hovermode='x unified' while axis tick labels remain unchanged, with coverage for the demonstrated case.