Попробуйте:
df['SumCount'] = df.groupby('Year')['Count'].transform('sum')
ax=df.plot(kind='bar', x='Year',y='Line', alpha=.7, color='g', position=1, width =.2)
df.plot(kind='bar', x='Year',y='SumCount', ax=ax, secondary_y=True, alpha=.8, position=0, width=.2)
Вывод:
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