So far, we've been focusing on marks and channels. As we have seen, they are really, really important components of every visualization. But it's also important to realize that marks and channels are not the only graphical elements that you typically have in a visualisation. There are a number of components that I like to call contextual components that are actually really, really important and really useful. If you remove them, interpreting a visualization gets really, really hard. So, I like to group all these contextual elements into two main sets. The first one is the set made of legends, labels, and annotations. The second one is axis, grids, and reference lines. So, let me start from the first group, legends, labels, and annotations. Why are they important? Well, they are important because they enable the interpretation of the graphical elements. Think about it. If you don't have legends, labels, or annotations, there is no way for you to actually interpret the meaning of the objects that are presented in the visualization. Let me give you a few examples. If I show you a heatmap matrix like this one, you see there's no labels, there's no legend, there's nothing here that actually helps you in the interpretation of the marks and the channels. Now, if I add some labels, the labels that explain what is in rows and columns, now you start understanding some of it. So, you know that in columns, we have different regions, and in rows, we have different categories of products, okay? This is very helpful. If I add a legend, the element that you see on the top right, now you also know that the color represents the average profit. So, all in a sudden, it becomes much easier to interpret the meaning of the graphical objects that you see in front of you, okay? Let me give you another examples. So, this is a set of timelines, okay? Color the timelines. Once again, there's no x-axis with labels, there's no y-axis with labels, there's no legend, there's no way for you to interpret this. Now, if I add axis, now you can start realizing and learning what these graph is about, right? So, you can see that on the x-axis, there are hours of the day, and on the y-axis, there are number of pedestrians injured. This is once again the dataset about vehicle collisions that we use previously in the course. So, these two elements are very important. If I add a legend, now you know that these different colors actually represent different months of the year. So, without these elements, it's basically impossible to interpret this graph. So, these are very important elements.