Why am I getting a line shadow in a seaborn line plot?

Here is the code:

fig=plt.figure(figsize=(14,8))
sns.lineplot(x='season', y='team_strikerate', hue='batting_team', data=overall_batseason)
plt.legend(title = 'Teams', loc = 1, fontsize = 12)
plt.xlim([2008,2022])

And here is the image

Just to let you know, I've already drawn another similar lineplot above this one.

There is line shadow showing the confidence interval, because the dataset contains multiple y(team_strikerate) values for each x(season) value. By default, sns.lineplot() will estimate the mean by aggregating over multiple y values at each x value.

After aggregation, the mean of y values at each x value will be plotted as a line. The line shadow represents the 95% confidence interval of the estimate.

To remove the line shadow, you can pass the argument ci=None to sns.lineplot(). (credit to @JohanC for providing this idea in this question's comment)

To change the confidence interval, you can pass the argument errorbar=('ci', <int>) to sns.lineplot().