df.plot() fails to plot multiple scenario quantiles

[zyankarli]

To reproduce:
get upper and lower quantiles of a pyam.IamDataFrame using compute.quantiles()

upper = df.filter(
        **{"variable": "Emissions|Kyoto Gases",
           "year": [2020, 2025, 2030, 2035, 2040]}
    ).compute.quantiles([1.0])

lower = df.filter(
        **{"variable": "Emissions|Kyoto Gases",
        "year": [2020, 2025, 2030, 2035, 2040]}
    ).compute.quantiles([0.0])

band = upper.append(lower)

The resulting band dataframe should look like:
<class 'pyam.core.IamDataFrame'>
Index:

• model : Quantiles (1)
• scenario : 0.0, 1.0 (2)
  Timeseries data coordinates:
  region : World (1)
  variable : Emissions|Kyoto Gases (1)
  unit : Mt CO2-equiv/yr (1)
  year : 2020, 2025, 2030, 2035, 2040 (5)

Now run

band.plot()

As a result, only one out of two scenarios ("1.0") is plotted. This bug seems robust against changing scenario and model names of band.

WORKAROUND
Re-defining band as a pyam.IamDataFrame again will allow correct plotting:

band_df = pyam.IamDataFrame(band)
band_df.plot()