Source: US Gallup Poll Stack Plot, Creative Commons: http://commons.wikimedia.org/w/index.php?title=File:USA_Gallup_abortion_opinion_poll_stacked_area.svg&page=1
Hi. This tutorial covers a certain type of graph called a stack plot. So let's take a look at a stack plot to begin with. So the following graph, a stack plot, shows the results from a US Gallup poll on the opinions about abortion. So if we take a look at just what this graph is showing to begin with is notice we of years on the bottom. So the scale is going-- we're going up by 3 years here. So it starts in '76, ends in 2009. And then on the y-axis, we're given percent.
And the data is broken up into, really, four categories. So this would be abortion being legal under any circumstance is the blue. The yellow is legal under certain circumstances. Green is illegal in all circumstances. And then the orange one here isn't labeled, but that was no opinion on the topic.
All right, so a couple of things to take a look at. Notice that it goes up to 100% for all of the years, and that's going to be pretty common when you're dealing with percent data. But it's important that you understand what's being shown here. So let's take a look at the green category, illegal in all circumstances.
So if you take a look up at the top here, in '76, that's at about 98%. Now, that doesn't mean that 98% of people thought that it was illegal in all circumstances. What that tells you-- if we wanted to actually figure out the percent who thought that way, we'd need to look at this vertical distance between here, the bottom of the region, and the top of the region.
So the bottom of the region, that's maybe about 77%. The top of the region is maybe about 98%. So that would seem to be a difference of about 21% there. So just because it goes up that high doesn't mean that 98% of people feel this way. Again, it would be more about 21%. So you need to be a little bit careful when you're interpreting these types of graphs.
So now, to formally define stack plot, a stack plot is a composite chart that shows two or more related data sets. Colors in a stack plot often distinguish different populations. Stack plots are generally line charts or bar graphs. So the one we looked at was a line chart. And then, again, when interpreting a stack plot, be sure that you understand that the heights of the bars in a bar graph or the dots in a line graph represent. So make sure you understand what those heights represent.
So again, going back to this example, if we're looking at this piece of data here, again, that doesn't mean that in 2004, about 75% of people thought it was legal under certain circumstances. Remember, in order to actually figure that out, you have to figure out what this vertical height is from 2004 here up to here.
The nice thing about a stack plot is you can see how multiple values change, so how all four categories are changing over time. You can see how they change just within that category. And then you can also compare them within other categories.
We also said that a stack plot can also be with a bar graph. So if we take a look now at the second example, an after school arts program for fifth grade students has run for the past three years. The gender of students for each roster is displayed in the stack plot below.
So notice we have our three years, year 1, year 2, year three. Now, the darker region represents the female. The lighter represents the male. So if we look in year 1, it seems like there were about 20 male.
Now, what the top of this bar means that's at about 45. That doesn't mean that there were 45 females, OK? What that means is that there is 45 minus 20 females. So there was about 25 females that's represented here and 20 males. So again, it's really important that you understand what the height represents.
And now, this is in frequency. The last one is percent. If it's in frequency, they won't always go to the top. Again, if we look at year 3, that means we have about 20, 21, 22 males. Again, the top looks to be about 51. So that would mean there would be about 30 females represented here.
So again, stack plots give you some good information. But make sure that you're not misled by the stack plot. Make sure you understand what it's showing. So that has been the tutorial on stack plots. Thanks for watching.