Generate image of the chart

[anuj9196]

I am using Django, but I do not want to use javascript to render the chart. I just want to show the chart image in the file.

Does this library support generating images in any format (SVG, jpg, png)

Tried using get_html_template() and get_python_template() but none of them are useful.

[nicohlr]

Hello @anuj9196 , thanks for using ipychart :) I think I can add a get_image() method to the Chart class, I'll try to do it quickly as soon as I have some free time to work on it.

[nicohlr]

Hello @anuj9196,

Apologies for the delayed response. I’ve added a to_image() method to the Chart class in the last version of ipychart (0.5.2), which allows you to generate and display chart images directly in your files without the need for JavaScript.

Please let me know if this works for you. If everything is resolved, I’ll go ahead and close this issue. Don’t hesitate to reach out if you have any further questions!

[tmolbergen]

Having a hard time using this function, files never seem to be stored (same entries as from other issue)

mychart = Chart(kind=kind, data=data, options=options, colorscheme=colorscheme)
    path = (os.path.dirname(os.path.abspath(__file__)))
    fullpath = f"{ path }/5days.png"
    print (fullpath)
    mychart.to_image(path=fullpath)

I was also wondering if it would be hard to implement a function which just outputs the chart in an object?

Similar to how pyWand would do it? - Note the png_bin part where the png is stored into the object and can then reused for other purposes (in my case sending an image over discord)

with Image(filename=svgname, format="svg") as img:
        img.format = 'png'
        png_bin = img.make_blob('png')

[nicohlr]

Explanation of How ipychart Works

ipychart is an ipywidget that provides a Python interface for creating and displaying charts using the JavaScript library, Chart.js. Essentially, ipychart acts as a bridge between Python and JavaScript, allowing users to create interactive charts directly in an IPython environments.

When you create a chart using ipychart, the chart is rendered in the notebook using the underlying JavaScript from Chart.js. The chart only exists in the browser’s memory as part of the interactive widget displayed in the notebook.

Why the to_image() method do not work in your specific case

When using the to_image() method in ipychart, there is an important detail to keep in mind: this method only works if the chart has already been rendered and displayed by the JavaScript part of the library. The chart must exist in the browser’s DOM (Document Object Model) for the to_image() method to capture it and convert it to an image.

In your specific case, you are attempting to use ipychart in a non-interactive environment (Flask backend), where the chart is not actually being displayed. You’re exporting the chart to HTML to be served on a web page using Flask, and then trying to use the to_image() method to generate an image. Since the chart hasn’t been displayed yet (it’s not in the browser’s DOM), the to_image() method is unable to capture and save the image.

Alternative approach to export a Chart as an image from a Flask app

Since you’re embedding the chart in an HTML page from within your Flask Python backend, you can use Chart.js’s native .toBase64Image() method in the JavaScript code of your Flask frontend to capture the chart once it has been rendered on the page.

You can do this by accessing the chart from JS through the canvas element (something like the CSS we use in your issue #11, but using JavaScript instead). You can then send this image back to the server or use it as needed.

That's actutally how the to_image() method works, but calling from the JavaScript code of your Flask app allows to export the image AFTER the rendering of the chart (while calling the to_image() method from Flask backend tries to export the image BEFORE the rendering of the chart).

Implementing a PNG Blob Output Function

Your idea of implementing a function that outputs the chart as an object (similar to pyWand) would require significant modification to ipychart.

Indeed, the current design of ipychart heavily relies on the interactive display of charts (as it is an ipywidget that involves both a Python part and a JavaScript part), and adding a feature to directly output an image blob without rendering the chart first would require changes to both the Python and JavaScript components of the library.

@tmolbergen Hope this helps you.

[nicohlr]

By reusing the example from issue #11, you can extend the existing setup to save the chart as an image file from the Flask frontend using the .toBase64Image() method. Here’s how you can do it:

index.html:

<!doctype html>
<html lang="en">
    <head>
        <title>Chart Example</title>
        <style>
            .chart-container canvas {
                height: 200px !important; /* This is the line where you change the height */
            }
        </style>
    </head>
    <body>
        <h1>My Chart</h1>
        <div class="chart-container">{{ chart | safe }}</div>
        <button id="exportChart">Export Chart as Image</button>

        <script>
            document.addEventListener("DOMContentLoaded", function () {
                const exportButton = document.getElementById("exportChart");

                exportButton.addEventListener("click", function () {
                    // Access the chart created by ipychart and exported as HTML
                    const chartElement = document.querySelector(
                        ".chart-container canvas",
                    );
                    const chart = Chart.getChart(chartElement);

                    // Export the chart as image using Chart.js toBase64Image
                    const imageBase64 = chart.toBase64Image();

                    // Send the image data to the Flask backend
                    fetch("/save-image", {
                        method: "POST",
                        headers: {
                            "Content-Type": "application/json",
                        },
                        body: JSON.stringify({ imageData: imageBase64 }),
                    })
                        .then((response) => response.json())
                        .then((data) => {
                            if (data.success) {
                                alert("Image saved successfully!");
                            } else {
                                alert("Failed to save image.");
                            }
                        })
                        .catch((error) => {
                            console.error("Error:", error);
                        });
                });
            });
        </script>
    </body>
</html>

app.py:

import base64
import os

from flask import Flask, jsonify, render_template, request
from ipychart import Chart

app = Flask(__name__)

@app.route("/")
def index():
    kind = "line"

    data = {
        "labels": [
            "Germany",
            "Spain",
            "UK",
            "Italy",
            "Norway",
            "France",
            "Poland",
            "Portugal",
            "Sweden",
            "Ireland",
        ],
        "datasets": [
            {
                "data": [14, 106, 16, 107, 45, 133, 19, 109, 60, 107],
                "lineTension": 0.3,
            }
        ],
    }

    options = {
        "maintainAspectRatio": False,
        "scales": {
            "x": {
                "title": {"display": True, "text": "Date"},
            },
            "y": {"title": {"display": True, "text": "Liter"}},
        },
        "animation": {"duration": 200},
    }
    colorscheme = "brewer.DarkTwo3"
    mychart = Chart(
        kind=kind, data=data, options=options, colorscheme=colorscheme
    )

    return render_template("index.html", chart=mychart.get_html_template())

@app.route("/save-image", methods=["POST"])
def save_image():
    data = request.get_json()
    image_data = data["imageData"]
    image_data = image_data.split(",")[1]
    image_bytes = base64.b64decode(image_data)
    image_path = os.path.join(os.getcwd(), "chart_image.png")

    with open(image_path, "wb") as f:
        f.write(image_bytes)

    return jsonify({"success": True})

if __name__ == "__main__":
    app.run(debug=True)

[tmolbergen]

Thanks alot for the detailed explanation, i might have to go back to the drawing board on how to generate a graph in the backend. I've tried various methods using headless browser sessions and so on, while I have managed to generate a graph on the backend, it seems somewhat heavy to spin up a chromium based webbrowser to take a snapshot :D

Yet again, thanks alot for taking the time to explain stuff which might be obvious for developers.