geom_step: confusing x-axis tooltip on a multi-group plot

```python
import pandas as pd
from lets_plot import *
LetsPlot.setup_html()
df = pd.read_csv("https://raw.githubusercontent.com/JetBrains/lets-plot-docs/master/data/iris.csv")
ggplot(df, aes(x="sepal_length", color="species")) + stat_ecdf()
```

The axis tooltip is displayed only for the first group of data. The other groups display the main tooltip not associated with the axis tooltip, which is confusing.:

Run the provided Python example with the iris data and inspect the axis and main tooltips for each species group. Trace the tooltip behavior for the multi-group stat_ecdf plot; done means every group displays an axis tooltip associated with its main tooltip.

Written by the indexing model from the issue text.