[BUG] etna.analysis.backtest does't visualize the forecast for a horizon equal to one

[nikolskydn]

🐛 Bug Report

The etna.analysis.backtest does't visualize the forecast for a horizon equal to one.

Expected behavior

For a horizon equal to one, a marker (from the label) should be displayed.

How To Reproduce

import numpy as np
import pandas as pd
from etna.datasets import TSDataset
from etna.transforms import SegmentEncoderTransform
from etna.pipeline import Pipeline
from etna.models import LinearPerSegmentModel
from etna.metrics import SMAPE
from etna.analysis import plot_backtest


freq = 'M'
idx = pd.to_datetime(pd.date_range(start='2022-06-01', end='2023-01-01', freq=freq))
df0 = pd.DataFrame({
    'timestamp': idx,
    'segment': ['user_0']*idx.size,
    'target': np.linspace(1, 21, num=idx.size)
})
df = TSDataset.to_dataset(df0)
ts = TSDataset(df=df, freq=freq)
model = LinearPerSegmentModel()
pipe = Pipeline(model=model, transforms=[SegmentEncoderTransform()], horizon=1)
metrics, forecast, info = pipe.backtest(ts=ts, metrics=[SMAPE()], n_folds=1)
plot_backtest(forecast_df=forecast, ts=ts, history_len=3)

Environment

print(platform.platform())
print(platform.python_version())

Linux-5.19.0-32-generic-x86_64-with-glibc2.35

3.10.6

! pip3 list | grep etna

etna 1.15.0

Additional context

The forecast is displayed if add a marker in the operators plot() from the function plotters.plot_backtest (when horizon=1).

Checklist

• Bug appears at the latest library version