diff --git a/pymc_marketing/mmm/plot.py b/pymc_marketing/mmm/plot.py
index b7a2ddae..e461b765 100644
--- a/pymc_marketing/mmm/plot.py
+++ b/pymc_marketing/mmm/plot.py
@@ -243,6 +243,40 @@ def _build_subplot_title(
             return ", ".join(title_parts)
         return fallback_title
 
+    def _align_y_axes(self, ax, ax2, include_zero=False):
+        """Align y=0 of primary and secondary y-axis."""
+        if ax.axes.get_ylim()[0] < 0 or ax2.axes.get_ylim()[0] < 0:
+            ylims1 = ax.axes.get_ylim()
+            ylims2 = ax2.axes.get_ylim()
+            # Find the ratio of negative vs. positive part of the axes.
+            if ylims1[1]:
+                ax1_yratio = ylims1[0] / ylims1[1]
+            else:
+                # Fully negative axis.
+                ax1_yratio = -1
+
+            if ylims2[1]:
+                ax2_yratio = ylims2[0] / ylims2[1]
+            else:
+                # Fully negative axis, may need to reflect the other
+                ax2_yratio = -1
+
+            # Make axis adjustments. If both axes fully negative, no adjustment.
+            if ax1_yratio < ax2_yratio:
+                ax2.set_ylim(bottom=ylims2[1] * ax1_yratio)
+                if ax1_yratio == -1:
+                    # if the axis is fully negative, center zero.
+                    ax.set_ylim(top=-ylims1[0])
+            elif ax2_yratio < ax1_yratio:
+                ax.set_ylim(bottom=ylims1[1] * ax2_yratio)
+                if ax2_yratio == -1:
+                    # if the axis is fully negative, center zero.
+                    ax2.set_ylim(top=-ylims2[0])
+        elif include_zero:
+            # Ensure both axes start at zero
+            ax.set_ylim(bottom=0)
+            ax2.set_ylim(bottom=0)
+
     def _get_additional_dim_combinations(
         self,
         data: xr.Dataset,
@@ -1190,6 +1224,9 @@ def _plot_budget_allocation_bars(
         ax.set_xticklabels(channels)
         ax.tick_params(axis="x", rotation=90)
 
+        # Ensure that y=0 are aligned between ax and ax2.
+        self._align_y_axes(ax, ax2, include_zero=True)
+
         # Turn off grid and add legend
         ax.grid(False)
         ax2.grid(False)