Create dumbbell-plots.md

Author: LiamConnors

doc/python/dumbbell-plots.md

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+---
+jupyter:
+  jupytext:
+    notebook_metadata_filter: all
+    text_representation:
+      extension: .md
+      format_name: markdown
+      format_version: '1.3'
+      jupytext_version: 1.14.1
+  kernelspec:
+    display_name: Python 3 (ipykernel)
+    language: python
+    name: python3
+  language_info:
+    codemirror_mode:
+      name: ipython
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+    mimetype: text/x-python
+    name: python
+    nbconvert_exporter: python
+    pygments_lexer: ipython3
+    version: 3.8.0
+  plotly:
+    description: How to create dumbbell plots in Python with Plotly.
+    display_as: basic
+    language: python
+    layout: base
+    name: Dumbbell Plots
+    order: 19
+    page_type: example_index
+    permalink: python/dumbbell-plots/
+    thumbnail: thumbnail/dumbbell-plot.jpg
+---
+
+## Basic Dumbbell Plot
+
+
+Dumbbell plots are useful for demonstrating change between two sets of data points, for example, the population change for a selection of countries for two different years
+
+In this example, we compare life expectancy in 1952 with life expectancy in 2002 for countries in Europe.
+
+```python
+import plotly.graph_objects as go
+from plotly import data
+
+import pandas as pd
+
+df = data.gapminder()
+df = df.loc[(df.continent == "Europe") & (df.year.isin([1952, 2002]))]
+
+countries = (
+    df.loc[(df.continent == "Europe") & (df.year.isin([2002]))]
+    .sort_values(by=["lifeExp"], ascending=True)["country"]
+    .unique()
+)
+
+data = {"x": [], "y": [], "colors": [], "years": []}
+
+for country in countries:
+    data["x"].extend(
+        [
+            df.loc[(df.year == 1952) & (df.country == country)]["lifeExp"].values[0],
+            df.loc[(df.year == 2002) & (df.country == country)]["lifeExp"].values[0],
+            None,
+        ]
+    )
+    data["y"].extend([country, country, None]),
+    data["colors"].extend(["green", "blue", "brown"]),
+    data["years"].extend(["1952", "2002", None])
+
+fig = go.Figure(
+    data=[
+        go.Scatter(
+            x=data["x"],
+            y=data["y"],
+            mode="lines",
+            marker=dict(
+                color="grey",
+            ),
+        ),
+        go.Scatter(
+            x=data["x"],
+            y=data["y"],
+            mode="markers+text",
+            marker=dict(
+                color=data["colors"],
+                size=10,
+            ),
+            hovertemplate="""Country: %{y} <br> Life Expectancy: %{x} <br><extra></extra>""",
+        ),
+    ]
+)
+
+fig.update_layout(
+    title="Life Expectancy in Europe: 1952 and 2002",
+    width=1000,
+    height=1000,
+    showlegend=False,
+)
+
+fig.show()
+
+```
+
+## Dumbbell Plot with Arrow Markers
+
+*Note: The `arrow`, `angleref`, and `standoff` properties used on the `marker` in this example are new in 5.11*
+
+In this example, we add arrow markers to the plot. The first trace adds the lines connecting the data points and arrow markers. 
+The second trace adds circle markers. On the first trace, we use `standoff=8` to position the arrow marker back from the data point.
+For the arrow marker to point directly at the circle marker, this value should be half the circle marker size. 
+
+```python
+import pandas as pd
+import plotly.graph_objects as go
+from plotly import data
+
+df = data.gapminder()
+df = df.loc[(df.continent == "Europe") & (df.year.isin([1952, 2002]))]
+
+countries = (
+    df.loc[(df.continent == "Europe") & (df.year.isin([2002]))]
+    .sort_values(by=["lifeExp"], ascending=True)["country"]
+    .unique()
+)
+
+data = {"x": [], "y": [], "colors": [], "years": []}
+
+for country in countries:
+    data["x"].extend(
+        [
+            df.loc[(df.year == 1952) & (df.country == country)]["lifeExp"].values[0],
+            df.loc[(df.year == 2002) & (df.country == country)]["lifeExp"].values[0],
+            None,
+        ]
+    )
+    data["y"].extend([country, country, None]),
+    data["colors"].extend(["silver", "lightskyblue", "white"]),
+    data["years"].extend(["1952", "2002", None])
+
+fig = go.Figure(
+    data=[
+        go.Scatter(
+            x=data["x"],
+            y=data["y"],
+            mode="markers+lines",
+            marker=dict(
+                symbol="arrow", color="black", size=16, angleref="previous", standoff=8
+            ),
+        ),
+        go.Scatter(
+            x=data["x"],
+            y=data["y"],
+            text=data["years"],
+            mode="markers",
+            marker=dict(
+                color=data["colors"],
+                size=16,
+            ),
+            hovertemplate="""Country: %{y} <br> Life Expectancy: %{x} <br> Year: %{text} <br><extra></extra>""",
+        ),
+    ]
+)
+
+fig.update_layout(
+    title="Life Expectancy in Europe: 1952 and 2002",
+    width=1000,
+    height=1000,
+    showlegend=False,
+)
+
+
+fig.show()
+
+```