Let's see the first type of change that is triggered by a selection. By far, one of the most important kind of changes is highlighting. When you select something on the screen, you want to highlight it because you want to communicate the idea that these elements have been selected. If you don't highlight them, you won't be able to communicate the idea that these items have been selected. You just don't know what has been selected. So, how do you highlight these items on the screen? Well, there are a number of different ways. Let me give you a few examples of different kinds of highlights and selections. So, the first one is the one that I've shown you before or very similar to the one that I've shown you before. We have a scatter plot, we want to select a group of data items, I draw an area with lasso tool and those elements that are below or within this area are automatically selected. But we don't have to work with scatter plots all the time. I've given you all only examples with scatter plots. So, what is another possible example? Let's say that you have a graph like the one that you see in front of you and when I hover over one of the nodes, I want to highlight the note that is currently selected, but also all the nodes that are currently connected with the node that I selected. So, as you can see what is interesting here is that I'm selecting one node but I am highlighting as a consequence not only the node that I selected, but also the edges and the nodes that are connected to the selected node. So, this gives you an example of the fact that there is an action to select something and then there is a reaction of the visualization that can change according to what type of visualization you are using and also what are the specific needs of this visualization. Let me give you another example that is also interesting and shows you a different aspect. So, let's say that we have a matrix visualization and now we are not interested in highlighting one single element, but we want to be able to highlight an entire row or column. So, this is also another type of selection, but it's not a selection of a single item or a group of items, it's a selection of a whole column or a whole row, okay? So, here we are typically selecting all the values of an attribute and we can also select in general multiple attributes. So, these are all examples of selections and we have an action and a reaction and as I said you can select different elements, different items, different attributes and it can be a single item or a single attribute or a group of elements. So, another thing that we have to cover is how do you actually change the visual appearance of the objects that are selected? This is also a visualization problem because you want them to stand out. There are many, many possible ways, but here I just want to mention two common ways that you will see around very, very often. This doesn't mean that you can use other methods, but these are very common ones. So, the first one is to change color or the border of an element as you can see here in this representation. Another very common and powerful one is the one that I've shown you in the examples at the beginning and it's the one that by default is used in Tableau. When you select some elements on the screen, the other elements that are not selected are grayed out or made transparent. This is also very, very powerful. So, as a general node, you have to think about all these little details when you are implementing selections in visualization. So, you have to think about what is selected, how the selection reflects or interacts with other items or attributes if necessary, how to change the visual appearance of these element or the connected elements, and how to do it in a way that is evident and doesn't create any doubts in the view of what is selected. As I said before, you also want to be really, really careful on how you implement your selections, because you want to implement interactive methods that are recognized as defaults. You don't want to implement any fancy interaction selection method unless you have a really good reason to do it. So, my suggestion is to always look around, what are the most common methods to do that and trying to stick to that unless you have a very good reason to break the rules.
