Add Observable Plot chart demo

examples/visualizations/observable_plot/app.py

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+from fasthtml.common import *
+import numpy as np
+
+plot_js = """
+function createPlot(data) {
+    const plot = Plot.rectY(data, Plot.binX({y: "count"},{x: d => d.value,fill:"steelblue"})).plot();
+    const div = document.querySelector("#plot");
+    div.replaceChildren(plot);
+}
+
+// Set up initial load via HTMX
+htmx.on('htmx:afterSettle', evt => {
+    if (evt.detail.target.id === 'data-store') {
+        // The data is now properly JSON-encoded
+        const data = JSON.parse(evt.detail.target.textContent);
+        createPlot(data);
+    }
+});
+"""
+
+app, rt = fast_app(
+    hdrs=(Script(src="https://cdn.jsdelivr.net/npm/d3@7"),
+          Script(src="https://cdn.jsdelivr.net/npm/@observablehq/plot@0.6")))
+
+@rt
+def index():
+    return Div(
+        H1(A("Observable", href="https://observablehq.com/@observablehq/plot", target="_blank"), " Plot Demo"),
+        P("The data is randomly generated on the server and is fetched on initial page load."),
+        P("Try opening the browser developer tools and viewing the Network tab to see the data reponse for each http request."),
+        # On bytton click it sends a get request to the `get_data` route and puts the response in the `data-store` div 
+        Button("Fetch New Data", get=get_data, hx_target="#data-store", cls='btn btn-primary'),
+        # Store for the JSON chart data
+        Div(id="data-store", get=get_data, hx_trigger="load", hidden=True),
+        # Plot container
+        Div(id="plot"),
+        # Include the JavaScript for the plot
+        Script(plot_js)
+        )
+
+@rt
+def get_data():
+    # Generate sample data
+    data = [{"value": float(x)} for x in np.random.randn(100)]
+    # Return as proper JSON response
+    return JSONResponse(data)
+
+serve()

examples/visualizations/observable_plot/metadata.ini

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+[REQUIRED]
+ImageAltText=Demo of Observable Plot charts in FastHTML
+ComponentName=Observable Plot Bar Chart
+ComponentDescription=How to display an Observable Plot bar chart in FastHTML, by David Gwyer.