Unify hovertemplate title cannot be edited.

Hello

It seems that we cannot modify the title of the hovertemplate when the chart uses a unify hovermode whenever the axis has not a date type.

Here a short example with 2 scatter plots, the first one with a date axis and the second is linear:

import numpy as np
import pandas as pd
from plotly.subplots import make_subplots

dates = pd.date_range("2022-01-01", "2022-03-01")
values = np.random.randint(0, 10, len(dates))

fig = make_subplots(cols=2) 
fig.add_scatter(x=dates, y=values, row=1, col=1)
fig.add_scatter(x=values, y=values, mode="markers", row=1, col=2)

fig.update_traces(hovertemplate="%{y}<extra></extra>")
fig.update_xaxes(row=1, col=1, hoverformat=" ")
fig.update_xaxes(row=1, col=2, type="linear", hoverformat=" ")
fig.update_layout(hovermode="x unified")

fig.show()

If you force the second scatter to be a date type, the hovertemplate title is removed with the workaround hoverformat=" ". However, it only works for date axis.

Here some references/related issues:
https://github.com/plotly/plotly.py/issues/4278
https://community.plotly.com/t/customizing-text-on-x-unified-hovering/39440/18
https://community.plotly.com/t/pass-nothing-or-disable-x-axis-hoverformat/67496

plotly version = 5.18.0

使用 plotly 5.18.0 运行所提供的 Python 复现，并比较 date-axis 和 linear-axis 的情况。阅读相关 issue #4278 及链接的社区讨论；完成的标准是：linear axis 上的 unified hover 允许移除或自定义 hovertemplate 标题，而无需强制使用 date axis，并包含回归测试覆盖。