What does fig.add_vline label do?

In Plotly, the fig.add_vline function is used to add a vertical line to a plot. The label parameter within fig.add_vline specifies the text that will be displayed as part of the vertical line's hover information or legend, depending on how you configure the plot. Let's break it down:

When you use fig.add_vline, you're typically creating a vertical line to highlight a specific point or region on your chart. The label parameter helps you provide context or explanation for that line.

Here’s a more detailed look at what the label does:

1. Hover Information: The label text is often displayed when you hover over the vertical line in the interactive Plotly plot. This provides immediate context to the viewer about what the line represents.

2. Legend: If you configure your plot to show a legend, the label can be displayed in the legend, allowing users to understand what each vertical line signifies at a glance.

3. Clarity and Context: The primary purpose of the label is to give meaning to the vertical line. For example, if you're plotting stock prices, you might add a vertical line to mark the date of a significant event, and the label could explain what that event was.

Here's an example of how you might use fig.add_vline with a label:

import plotly.graph_objects as go

fig = go.Figure()

fig.add_trace(go.Scatter(x=[1, 2, 3, 4, 5], y=[10, 11, 12, 13, 14], mode='lines'))

fig.add_vline(x=3, line_width=3, line_dash="dash", line_color="green", label="Important Event")

fig.show()

In this example, a vertical line is added at x=3, and its label is set to "Important Event". When a user hovers over the line, the label will appear, explaining the significance of that point.

The label parameter is crucial for making your Plotly charts informative and accessible, especially when they contain annotations like vertical lines.

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