subplot で、ax の xtick を傾ける

fig,ax_ = plt.subplots(nrows=10, ncols=2, figsize=(14, 20))
ax_ = ax_.ravel()

for i in range(20):
    list_ = M_feature_inverse[i][:3]
    ax = ax_[i]
    for l in list_:
        all_df_tmp = all_df_.loc[all_df_['pk']==l, :].groupby('request_at_dt').size().reset_index(name='c')
        df_display_1 = all_df_tmp = all_df_tmp.sort_values(by='request_at_dt', ascending=False)

        ax.plot(list(df_display_1['request_at_dt']), list(df_display_1['c']), label=l+'_access_count')
    # ax.axvline(x='2018-09-01', linewidth=2, color='r')
    plt.sca(ax)
    plt.xticks(rotation=20)
    ax.legend(loc='upper left')

# fig.autofmt_xdate()
plt.tight_layout( )
fig.savefig('access_count_.png'.format(target))
fig.show()