Fix #66 via suggestion from @lisphilar

This commit fixes "Decreased legend visibility on `massive-plots`" (#66)
 via a suggestion by @lisphilar.

 Changelist:
 - Fix `epi.plots.native::stacked`

Co-authored-by: Lisphilar <7270139+lisphilar@users.noreply.github.com>

epispot/plots/native.py

@@ -162,6 +162,7 @@ def stacked(Model, time_frame, title='Compartment Populations over Time',
     if log:
         plt.yscale('log')
     plt.title(title)
-    plt.legend(loc='upper left', facecolor='white')
+    plt.legend(loc="lower center", bbox_to_anchor=(0.5, -0.2), borderaxespad=0, ncol=2)
+    plt.tight_layout()
 
     return plt