Streamlit Plotly Chart Width Warning Explained

Have you ever been working with Streamlit, happily plotting away with plotly_chart, only to be greeted by a rather alarming deprecation warning? You might have been trying to set the width of your chart using something like width="content", and suddenly, the console flashes a warning about kwargs. It’s a bit of a head-scratcher, especially when you feel like you’re only using the parameters exactly as the documentation suggests. You might be thinking, "But I'm not using any weird keyword arguments! What's going on?" Well, you're not alone, and this article is here to clear up that confusion.

Understanding the Deprecation Warning in Streamlit's plotly_chart

Let's dive right into it. The core of the issue lies in how Streamlit handles arguments passed to st.plotly_chart. Previously, some arguments that were meant for controlling the chart's appearance, like width and height, might have been indirectly passed through a mechanism that Streamlit flagged as variable keyword arguments, or kwargs. Even though you, as the user, were likely specifying these parameters explicitly and correctly according to the documented API, the internal workings of Streamlit might have been interpreting them in a way that triggered a warning about the use of kwargs. This is a common scenario when libraries evolve; internal implementations change, and sometimes these changes can lead to unexpected user-facing messages, even if the functionality remains the same from the user's perspective.

The warning message itself, "Variable keyword arguments for st.plotly_chart have been deprecated and will be removed in a future release. Use the config argument instead to specify Plotly configuration options," is a bit of a red herring in this specific case. It's a general warning designed to guide users away from passing arbitrary Plotly-specific configurations directly as kwargs to st.plotly_chart. Instead, Streamlit wants you to consolidate those Plotly settings within the config dictionary argument. However, when you use parameters like width="content" or height="auto", which are Streamlit-specific ways to control the chart's dimensions, these parameters aren't actually being treated as arbitrary Plotly kwargs. They are designed to work directly with st.plotly_chart.

Why the Warning Appears (and Why It's Not Exactly Your Fault)

The most probable reason for this specific warning appearing when using width="content" (or similar documented Streamlit parameters) is due to a change in Streamlit's internal argument handling. In versions prior to the fix, the way st.plotly_chart processed these specific width/height arguments might have inadvertently fallen into the kwargs detection logic. This means that even though you were providing a perfectly valid and intended argument, Streamlit's internal code detected it as something that could be a kwargs and thus issued the deprecation warning.

This is a classic example of how a library update can introduce an inadvertent regression. The functionality itself (setting the chart width) is not broken, but the warning message is misleading and can cause unnecessary concern for users. Streamlit is a fantastic tool for building data apps quickly, and the developers are constantly refining it. Sometimes, these refinements, particularly around how arguments are passed and validated, can lead to these kinds of slightly confusing situations. The intention behind the warning was good – to encourage cleaner, more organized configuration of Plotly charts – but its application in this specific context of Streamlit's own dimension arguments was a misfire. This is why the bug report explicitly states, "Yes, this used to work in a previous version," highlighting its nature as a regression.

The Fix: Cleaner Argument Handling

Fortunately, this issue has been addressed in subsequent versions of Streamlit. The fix involves refining how st.plotly_chart parses and handles its arguments. Developers have likely updated the internal logic so that Streamlit-specific arguments like width and height are correctly identified and processed, ensuring they do not fall into the kwargs detection trap. This means that when you use st.plotly_chart(fig, width="content") in a fixed version of Streamlit, you should no longer see that confusing deprecation warning. The library is now smarter about distinguishing between its own control parameters and arbitrary Plotly configurations being passed through.

How to Ensure You're Using It Correctly

When working with st.plotly_chart, it's always a good practice to be aware of how to pass configurations. For Plotly-specific settings that go beyond basic dimensions, the config dictionary is indeed the way to go. For example, if you want to enable the modebar, hide the zoom/pan tools, or customize other Plotly-level features, you'd pass them like this:

config = {
 "displaylogo": False,
 "modeBarButtonsToRemove": ["zoom2d", "pan2d"]
}
st.plotly_chart(fig, config=config)

For controlling the overall size and responsiveness of the chart within your Streamlit app, you'll continue to use the width and height arguments directly, like so:

st.plotly_chart(fig, width=600, height=400) # For fixed pixel sizes
st.plotly_chart(fig, width="content") # To adapt to container width
st.plotly_chart(fig, height="auto") # To adapt to chart height

The Importance of Clear Documentation and Bug Reporting

This situation underscores the critical importance of clear documentation and proactive bug reporting in software development. The original bug report was detailed, included a reproducible code example, and clearly stated the expected versus current behavior. This kind of information is invaluable to developers trying to pinpoint and fix issues. It also highlights the ongoing effort to keep libraries like Streamlit user-friendly and robust. Even when a warning seems misplaced, understanding its context and reporting it can lead to a better experience for everyone.

In essence, if you encountered this warning, it was likely a minor hiccup in Streamlit's argument parsing that has since been resolved. By understanding the distinction between Streamlit's own arguments (width, height) and Plotly configurations (passed via config), you can continue to build dynamic and visually appealing data applications with confidence. Always ensure you're using the latest stable version of Streamlit to benefit from these fixes and improvements.

For more information on Streamlit's charting capabilities and best practices, you can always refer to the official Streamlit Documentation. This is your go-to resource for understanding all the components and features Streamlit has to offer.