A triangular graph also known as a ternary plot shows the relationship
between three sets of variables. They are useful when free data sets combined add
up to 100%. For example, this diagram shows the
proportion of silica, titomagnetite and calcium carbonate in sand. To construct
the triangular graph we first need some data. I have some data for the primary,
secondary and tertiary sectors of an economy for 12 countries as shown in the
table on the screen.
Now first we need to draw an equilateral triangle. Each axis
needs to be divided into 100, representing of a percentage. Now we're
going to draw each axis 10 centimeters long. So first I'll draw of the bottom
horizontal axis 10 centimeters long, and from left to right I'm going to mark it
out in one centimeter intervals. And each centimeter is going to represent
ten percent. So we'll start with zero in the bottom left hand corner, and then go ten.
Now when we're done this, we are going to draw a vertical line from the 50 percent
mark here. Straight up the page and we're going to rub this line out later. The
purpose of this line is it's going to help us join our other two ten
centimeter lines. Now I'm going to draw two ten centimeter lines which are going
to run from the 0% and 100% marks of the horizontal axis, until they meet on this
vertical line that I drew in the earlier step. So, that's about 10 centimeters there.
And that there will be 10 centimeters on that side. We just got
a mark out of scale again, just like we did earlier.
Zero, 10, 20, 30 ,40, 50, 60, 70, 80 ,90, 100 percent, yeah!
Now on this side zero, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100 percent here.
We'll label access and
we'll make our horizontal one. (line) We'll make this the agriculture.
Make this one industry on the right here,
and on the left here make this services.
and now we can rub out this vertical line in the middle. We don't need any more.
Now I'm going to draw on three sets of parallel lines for all three axis, and they're
going to be drawn at a 60 degree angle. So that the 10% here will equal join up
the 90% on this line. So we'll carry these all the way across.
We'll do the other side.
And we'll also do these horizontal ones. Okay that looks nice. Now I'm ready to plot the
points for our 12 countries. I will do this by finding the point where all
three elements intersect. Now Angola for example has 10.2% employed in
agriculture. So what I'm going to do is I'm going to run my pen along the scale
for agriculture here, until it reaches the 10% mark, and that is there. Okay, Now
the next figure is 61.4% for industry. So what I'm going to do is I'm
going to run my other pen here along the scale for industry on the right here
until it reaches 61%. And that is about there. Now what I'm going to do is I'm
going to use both pens and I'm going to follow the lines until they find a point
at which they both intersect. Now watch this!
Intersects about here, okay. So now what I'm going to do is I'm going to just put
a little star, little cross there and I will write ANG for Angola, which I'll
later put in a key, okay. Now if I follow this line here to the services scale,
you'll see that it roughly comes to about 28%. Now if I add that 28% to the 10% and
the 61% that I did earlier, it of course comes to 100%.
Now I will plot the data for the other 11 countries.
Now I've almost finished my graph and I've added a key
on the right hand side. Now I'm going to give it an appropriate title.
We're going to call this a triangular graph showing
GDP composition by sector.
Now here is my completed graph