Sorting values after transpose of dataframe

My dataframe is:

After transposing

df=df.set_index(0)
print(df)
df = df.T
print(df)
df = df.sort_values(by=df.index, ascending=True)
print(df)

But I am getting an error like:

KeyError: Int64Index([1, 2, 3, 4], dtype='int64')

I am trying to plot index values to a column but it should be in ascending order because index 3 refers new value but index 1 is an old value in the time range.

fig = px.line(df, x=df.index, y=df['a'])
fig.show()

>Solution :

Instead of df.sort_values(by=df.index), use df.sort_index:

df = df.sort_index(ascending=True)

Output:

>>> df
      1     2      3
a  1091   347   2164
b   208   284  27647
c     0  8126     22