Customizing the X-axis in Dygraph: Using a Weekly Ticker

Introduction

In this article, we will explore how to use a custom ticker function in Dygraph to label the x-axis. Specifically, we will demonstrate how to create a weekly ticker that aligns with Mondays.

Dygraph is a popular JavaScript library for creating interactive charts and graphs. One of its features is automatic time axis scaling, which can be convenient when working with date-based data. However, this default behavior may not always meet the requirements of our analysis. In this article, we will show how to create a custom ticker function that aligns with Mondays.

Prerequisites

Before diving into the solution, it’s essential to have some basic knowledge of R and Dygraph. Specifically:

Familiarity with the xts package for time series data manipulation.
Understanding of the dygraphs library in R.

If you’re new to Dygraph or need more information on how to get started with it, we recommend checking out the official Dygraph documentation.

The Problem

The question from Stack Overflow highlights a common issue when working with weekly data in Dygraph. By default, the x-axis is aligned with Sundays, which may not be ideal for analysis or visualization purposes.

To resolve this issue, we can create a custom ticker function that starts on the first day of our data and places a tick every seven days, aligning with Mondays.

The Solution

The solution involves creating a custom ticker function using R’s dygraph library. We’ll use the DYGRAPH_TICKER_FUNC macro to define a JavaScript function that will be executed by Dygraph during rendering.

Here’s an example of how you can create this custom ticker function:

## Create a custom ticker function

The `DYGRAPH_TICKER_FUNC` macro in Dygraph allows us to define a JavaScript function that will be executed during rendering.
```javascript
function(start_time, end_time, pixels, opts, dygraph, vals) {
  // Get the first day of our data as the start time
  var start = new Date(vals[0][1]);
  
  // Set the spacing between ticks (7 days in milliseconds)
  var spacing = 1000 * 604800;
  
  // Create an array to store the tick labels
  var ticks = [];
  
  // Loop through each day and create a tick label
  for (var t = start; t <= end_time; t += spacing) {
    // Get the formatted date string using the weekly formatter
    var label = dygraph.options("axisLabelFormatter")(new Date(t), Dygraph.WEEKLY, opts, dygraph);
    
    // Add the tick label to our array
    ticks.push({v: t, label: label});
  }
  
  // Return the array of tick labels
  return ticks;
}

Applying the Custom Ticker Function

Once we’ve defined our custom ticker function, we can apply it to our Dygraph chart using the dyAxis method:

## Apply the custom ticker function to our Dygraph chart

Now that we have a custom ticker function, let's see how we can use it in our Dygraph chart.

```r
# Load required libraries
library("magrittr")
library("dygraphs")

# Create sample data
daily <- structure(c(4000, 5000, 3000, 7000, 2000, 5000, 7000, 
                   2000, 3000, 6000, 5000, 9000, 2000, 2000, 2000, 
                   7000, 9000, 2000, 1000, 13000), .Dim = c(10L, 2L), .Dimnames = list(
                     NULL, c("col1", "col2")), index = structure(c(1476032400, 
                                                                   1476637200, 1477242000, 1477846800, 1478451600, 1479056400, 1479661200, 
                                                                   1480266000, 1480870800, 1481475600), tzone = "Asia/Saigon", tclass = c("POSIXct", 
                                                                                                                                          "POSIXt")), class = c("xts", "zoo"), .indexCLASS = c("POSIXct", 
                                                                                                                                                                                               "POSIXt"), tclass = c("POSIXct", "POSIXt"), .indexTZ = "Asia/Saigon", tzone = "Asia/Saigon")

# Create the Dygraph chart with our custom ticker function
dygraph(daily, main = "Stackoverflow") %&gt;%
  dyRangeSelector() %&gt;%
  dyAxis("x", ticker='function(start_time, end_time, pixels, opts, dygraph, vals) { ... }') %&gt;%
  dyOptions(useDataTimezone = TRUE)

Conclusion

By creating a custom ticker function using R’s dygraph library, we can align the x-axis with Mondays and meet our specific requirements. This example demonstrates how to use this feature in practice and provides a starting point for further customization of your Dygraph charts.

Remember to check out the official Dygraph documentation for more information on using custom ticker functions and other advanced features.

Last modified on 2023-12-08