Fixing Chartkick Y-Axis Label Formatter Issues in Rails 7

Customizing Chartkick Charts with Javascript Functions in Rails 7

Chartkick is a fantastic tool for visualizing data in Rails applications, making it simple to generate interactive charts with minimal code. However, incorporating custom JavaScript functions within Chartkick options can sometimes lead to challenges, especially when dealing with more advanced configurations.

One common use case is customizing the y-axis labels by applying a JavaScript function to format the numbers. This can be helpful when you want to display data in a particular format, such as rounding numbers or adding a unit of measurement. In Rails 7, achieving this requires careful handling of JavaScript within Ruby templates.

Although the default Chartkick setup works fine, issues can arise when introducing a JavaScript formatter in the y-axis options. A common error involves the undefined local variable, causing confusion about how to properly integrate the JavaScript function.

In this article, we’ll explore how to resolve the issues you encounter when embedding JavaScript in Chartkick options. We'll walk through common mistakes, provide code solutions, and ensure that your chart renders with the correctly formatted y-axis labels.

Understanding Chartkick and JavaScript Integration in Rails 7

When integrating Chartkick with Rails 7, it’s important to understand how Chartkick operates with dynamic JavaScript functions. In the basic example provided, we used the line_chart helper to create a simple chart. The option defer: true is critical here because it tells the page to load the chart only after all DOM elements and JavaScript files are fully available. This is especially useful for pages that might load content dynamically or have large datasets. Without deferring the load, the chart might attempt to render before the necessary elements are in place, leading to errors.

The next step involved formatting the y-axis labels. This is where embedding a JavaScript function in the chart options comes into play. Normally, Ruby and Rails try to escape any potentially unsafe characters in strings to prevent cross-site scripting (XSS) attacks. This is where the raw() function becomes essential. By wrapping the JavaScript function in raw(), we ensure that the function is outputted exactly as written, without being altered by Rails’ security mechanisms. However, simply embedding the raw JavaScript function isn’t enough on its own, as we saw with the TypeError in the console.

To address this error, the second approach involved better error handling and a modular structure. The use of the Chartkick.eachChart function ensures that all charts on the page can be iterated over and redrawn, making this a versatile solution for applications with multiple charts. This approach not only makes the chart rendering more reliable but also allows for more flexibility if changes to the chart configuration or data are needed after the initial load. Additionally, by catching any errors that may occur during the chart rendering process using console.error(), we ensure that errors are logged without crashing the entire page.

Finally, for more advanced control, integrating Chart.js through Chartkick allows developers to take full advantage of Chart.js' customization options. This method is ideal for more complex scenarios where you need detailed control over chart configurations, such as customizing the yaxis labels with unit symbols or other specific formatting. By using Chart.js' callback functions, we can further manipulate how data is presented to the user, offering more flexibility than what the standard Chartkick options might allow. This approach provides a powerful way to enhance user experience by ensuring that data is not only accurate but also displayed in a meaningful way.

<%# Back-end: Rails view with embedded JavaScript for Chartkick options %>
<%= line_chart [{name: "Weather forecast", data: @dataset}],
           { defer: true,
             yaxis: { labels: { formatter: raw("function(val, opts) { return val.toFixed(2); }") } } 
           } %>

<%# Front-end: Handling the chart rendering in JavaScript %>
<script>
document.addEventListener('DOMContentLoaded', function() {
  var chartElement = document.querySelector("[data-chartkick-chart]");
  if (chartElement) {
    Chartkick.eachChart(function(chart) {
      chart.redraw();
    });
  }
});
</script>

<%# Back-end: Define a helper for rendering chart with formatter %>
def formatted_line_chart(dataset)
  line_chart [{ name: "Weather forecast", data: dataset }],
            defer: true,
            yaxis: { labels: { formatter: raw("function(val, opts) { return val.toFixed(1) + '°C'; }") } }
end

<%# In your view %>
<%= formatted_line_chart(@dataset) %>

<%# Front-end: Improved error handling for chart rendering %>
<script>
document.addEventListener('DOMContentLoaded', function() {
  try {
    Chartkick.eachChart(function(chart) {
      chart.redraw();
    });
  } catch (e) {
    console.error("Chartkick Error:", e.message);
  }
});
</script>

<%# Back-end: Rails view calling a custom JavaScript function for full Chart.js control %>
<%= line_chart [{name: "Weather forecast", data: @dataset}],
           library: { scales: { yAxes: [{ ticks: { callback: "function(value) { return value + ' units'; }" } }] } } %>

<%# Front-end: Manually handling chart instantiation with Chart.js via Chartkick %>
<script>
document.addEventListener('DOMContentLoaded', function() {
  var chartElement = document.querySelector("[data-chartkick-chart]");
  if (chartElement) {
    var chartData = JSON.parse(chartElement.dataset.chartkick);
    var chart = new Chartkick.LineChart(chartElement, chartData);
  }
});
</script>

Deep Dive into Rails 7 and Chartkick: Y-Axis Label Customization

In Rails 7, Chartkick remains one of the most powerful tools for integrating charts, but there are advanced customizations that require additional understanding. One such customization involves modifying the y-axis labels. Although Chartkick supports a variety of options, handling JavaScript functions within a Ruby template can be tricky due to how Rails processes strings and protects against XSS vulnerabilities. This makes embedding functions directly into chart options non-trivial and may lead to issues if not handled correctly.

Another important aspect to consider is Chart.js, which can be integrated via Chartkick. By using callback functions and raw(), we can format axis labels in more specific ways, adding units or modifying values dynamically. However, when working in Rails, especially with embedded JavaScript, Rails tends to escape any potentially dangerous characters. This is why using raw() to avoid unwanted escaping is crucial when inserting JavaScript into a Ruby template. Still, even after solving this, developers might run into browser errors such as “formatter is not a function,” requiring careful handling of the JavaScript execution flow.

Lastly, handling DOM events efficiently is crucial for chart rendering. For example, using the DOMContentLoaded event ensures that the charts are not rendered prematurely. This step prevents JavaScript from trying to manipulate elements that haven’t been fully loaded, which could otherwise lead to issues when redrawing the charts or rendering complex data visualizations. Ultimately, these aspects highlight the delicate interaction between Rails and JavaScript when using tools like Chartkick and Chart.js.