So, one important idea behind visualization is the idea that every single visualization can be described as a mapping between data properties and visual properties. Let's start with an example to explain that. So, this is a simple dataset, mostly a list coming from sales data of a company. This company sells a lot of different products and these products belong to different categories, and this list that you see on the left is all the categories of products that they sell. For each category, we calculated the total amount of sales that they made during the last year. So, this is a list last year, products and the total amount of sale for each product. How do you visualize that? Well, one way to visualize that is by using a bar chart. What is a bar chart? Well, it's just a chart that, again, it's a sort of list, but for every element of the list there is an associated bar that shows with the length of the bar the quantity that is represented in the table that I've just shown to you. So, when you look at a visual representation like this one what you can do is to identify the symbols and think about how these symbols represent objects of interest, objects that come from the visual representation. So, in this case, the objects are the categories of products and the symbols are bars. But, you also have graphical properties associated to the symbols. In this graphical properties represent properties of the objects. So, let's go back to the same graph. What do you see here? So, you have for every single bar represents one category. So, the symbol bar represents categories, and now you have that different visual channels or visual properties represent a number of different properties that are included in the data. So, in particular here, we can see that the categories are represented in terms of the position that they have in the list and the values that are associated to the categories are represented by the length of the bars. So, here we are using two visual properties, the position of the elements in the list to represent the elements, the categories, and the length of the bar to represent the quantity. So, these visual properties are actually the visual channels, what we call visual channels. So, visual channels are graphical properties that one can use to represent information visually. It turns out that there are many other possible visual properties, graphical properties that one can use in visualization to represent information. In the previous examples, I mentioned position and length of a bar, but there are many many other visual properties that one can use. Here is a summary table of some of the most popular properties that one can use. I've taken this table from the PhD thesis of Eamonn Maguire, who actually created a lot of really interesting and nice visuals to show how visualization works, and this is a summary table, as I said, that represents all the main visual properties that one can use. So, I want to walk you through some of them. For instance, we have size. One can use size to represent quantity. The bigger something is the more quantity it represents. One can use color intensity. So, the brighter something is and the higher the value it represents, or say, the slope of a line can represent a quantity, or different color, what we call color hues, which is basically the name of the colors, can represent different categories, and so on. So, there are many different visual channels and the main idea is that visual channels can be used to represent information that is contained in the data. Before I conclude, I want to give you another practical example. So, here is another graph taken from the famous Gapminder tool created by Hans Rosling to basically represent the temporal evolution of human conditions around the world. This specific graph is a scatter plot and it basically represents the relationship between income in different countries and life expectancy. Every single bubble here represents one country and the color represents different regions in the world and size of the bubble represents the population size. So, here, if you think about it, we have many different channels used at once. So, the symbols are the bubbles and each bubble, as I said, represents one country. But, now we have associated these bubbles a number of different visual properties. So, what are these properties? There is the X position of the bubble, there is the Y position of the bubble. These are already two visual channels, X position and Y position, then we have the size of the bubble. This is yet another channel that represents population size, and then we have color that represents different regions in the world. So, once again, these are examples of visual channels.
