Asynchronous data loading using queue
In this recipe, we will demonstrate another very useful technique commonly used to process or generate data in large data visualization projects. It is usually necessary in complex visualization project to load and merge multiple datasets from different sources before proceeding to visualizing. The challenge in this kind of asynchronous loading is the difficulty in waiting to know when all datasets have been successfully loaded since only then the visualization can begin. D3 provides a very convenient
queue interface to help organize these types of asynchronous tasks and helps you coordinate among them, which is the focus of this recipe.
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