diff --git a/demos/forecast_chart.py b/demos/forecast_chart.py
index c70a91bd..4951fa25 100644
--- a/demos/forecast_chart.py
+++ b/demos/forecast_chart.py
@@ -86,7 +86,7 @@ def _(np, pl):
 
 @app.cell
 def _(AltairWidget):
-    widget = AltairWidget(width=600, height=400)
+    widget = AltairWidget(height=400)
     return (widget,)
 
 
@@ -115,7 +115,6 @@ def _(df, fit_window, forecast_chart, projection_days, widget):
         fit_window=fit_window.value,
         projection_days=projection_days.value,
         title="Synthetic metric forecast",
-        width=600,
     )
     return (time,)
 
@@ -152,7 +151,6 @@ def _(df, fit_window, forecast_chart, projection_days, time):
         fit_window=fit_window.value,
         projection_days=projection_days.value,
         title="Synthetic metric forecast (plain)",
-        width=600,
     )
     return
 
diff --git a/wigglystuff/utils.py b/wigglystuff/utils.py
index ff496649..196b78ab 100644
--- a/wigglystuff/utils.py
+++ b/wigglystuff/utils.py
@@ -108,7 +108,7 @@ def forecast_chart(
     fit_window: int = 180,
     projection_days: int = 365,
     title: str | None = None,
-    width: int | None = None,
+    width: int | str | None = None,
     height: int = 400,
 ):
     """Create a time series chart with an exponential forecast.
@@ -123,7 +123,7 @@ def forecast_chart(
         fit_window: Number of most-recent data points to use for the fit.
         projection_days: How many days to project into the future.
         title: Optional chart title.
-        width: Chart width in pixels. If None, uses Altair default.
+        width: Chart width in pixels, or "container" for responsive. Defaults to "container".
         height: Chart height in pixels (default 400).
 
     Returns:
@@ -229,10 +229,9 @@ def forecast_chart(
 
     props = {
         "height": height,
+        "width": width if width is not None else "container",
         "usermeta": {"embedOptions": {"actions": False}},
     }
-    if width is not None:
-        props["width"] = width
     if title:
         props["title"] = title