mscroggs.co.uk
mscroggs.co.uk

subscribe

Blog

 2020-03-31 
Recently, you've probably seen a lot of graphs that look like this:
The graph above shows something that is growing exponentially: its equation is \(y=kr^x\), for some constants \(k\) and \(r\). The value of the constant \(r\) is very important, as it tells you how quickly the value is going to grow. Using a graph of some data, it is difficult to get an anywhere-near-accurate approximation of \(r\).
The following plot shows three different exponentials. It's very difficult to say anything about them except that they grow very quickly above around \(x=15\).
\(y=2^x\), \(y=40\times 1.5^x\), and \(y=0.002\times3^x\)
It would be nice if we could plot these in a way that their important properties—such as the value of the ratio \(r\)—were more clearly evident from the graph. To do this, we start by taking the log of both sides of the equation:
$$\log y=\log(kr^x)$$
Using the laws of logs, this simplifies to:
$$\log y=\log k+x\log r$$
This is now the equation of a straight line, \(\hat{y}=m\hat{x}+c\), with \(\hat{y}=\log y\), \(\hat{x}=x\), \(m=\log r\) and \(c=\log k\). So if we plot \(x\) against \(\log y\), we should get a straight line with gradient \(\log r\). If we plot the same three exponentials as above using a log-scaled \(y\)-axis, we get:
\(y=2^x\), \(y=40\times 1.5^x\), and \(y=0.002\times3^x\) with a log-scaled \(y\)-axis
From this picture alone, it is very clear that the blue exponential has the largest value of \(r\), and we could quickly work out a decent approximation of this value by calculating 10 (or the base of the log used if using a different log) to the power of the gradient.

Log-log plots

Exponential growth isn't the only situation where scaling the axes is beneficial. In my research in finite and boundary element methods, it is common that the error of the solution \(e\) is given in terms of a grid parameter \(h\) by a polynomial of the form \(e=ah^k\), for some constants \(a\) and \(k\).
We are often interested in the value of the power \(k\). If we plot \(e\) against \(h\), it's once again difficult to judge the value of \(k\) from the graph alone. The following graph shows three polynomials.
\(y=x^2\), \(y=x^{1.5}\), and \(y=0.5x^3\)
Once again is is difficult to judge any of the important properties of these. To improve this, we once again begin by taking the log of each side of the equation:
$$\log e=\log (ah^k)$$
Applying the laws of logs this time gives:
$$\log e=\log a+k\log h$$
This is now the equation of a straight line, \(\hat{y}=m\hat{x}+c\), with \(\hat{y}=\log e\), \(\hat{x}=\log h\), \(m=k\) and \(c=\log a\). So if we plot \(\log x\) against \(\log y\), we should get a straight line with gradient \(k\).
Doing this for the same three curves as above gives the following plot.
\(y=x^2\), \(y=x^{1.5}\), and \(y=0.5x^3\) with log-scaled \(x\)- and \(y\)-axes
It is easy to see that the blue line has the highest value of \(k\) (as it has the highest gradient, and we could get a decent approximation of this value by finding the line's gradient.

As well as making it easier to get good approximations of important parameters, making curves into straight lines in this way also makes it easier to plot the trend of real data. Drawing accurate exponentials and polynomials is hard at the best of times; and real data will not exactly follow the curve, so drawing an exponential or quadratic of best fit will be an even harder task. By scaling the axes first though, this task simplifies to drawing a straight line through the data; this is much easier.
So next time you're struggling with an awkward curve, why not try turning it into a straight line first.

Similar posts

Visualising MENACE's learning
World Cup stickers 2018, pt. 2
Close encounters of the second kind
Christmas (2020) is over

Comments

Comments in green were written by me. Comments in blue were not written by me.
 Add a Comment 


I will only use your email address to reply to your comment (if a reply is needed).

Allowed HTML tags: <br> <a> <small> <b> <i> <s> <sup> <sub> <u> <spoiler> <ul> <ol> <li>
To prove you are not a spam bot, please type "axes" in the box below (case sensitive):

Archive

Show me a random blog post
 2021 

May 2021

Close encounters of the second kind

Jan 2021

Christmas (2020) is over
 2020 
▼ show ▼
 2019 
▼ show ▼
 2018 
▼ show ▼
 2017 
▼ show ▼
 2016 
▼ show ▼
 2015 
▼ show ▼
 2014 
▼ show ▼
 2013 
▼ show ▼
 2012 
▼ show ▼

Tags

world cup inline code royal baby pascal's triangle recursion numbers curvature flexagons propositional calculus geometry data guest posts bodmas martin gardner error bars convergence weak imposition chebyshev rhombicuboctahedron pi chalkdust magazine games captain scarlet final fantasy stickers sound finite element method signorini conditions geogebra twitter craft tmip dates manchester science festival asteroids phd london underground matt parker hannah fry draughts raspberry pi dataset chess stirling numbers coins harriss spiral advent calendar arithmetic accuracy logs polynomials hats books countdown python folding tube maps ternary national lottery radio 4 london map projections nine men's morris javascript football weather station light cross stitch reuleaux polygons fractals probability rugby wool manchester approximation sorting european cup quadrilaterals big internet math-off golden ratio royal institution inverse matrices matrices numerical analysis computational complexity a gamut of games simultaneous equations binary boundary element methods speed gaussian elimination electromagnetic field mathsteroids dragon curves news data visualisation pizza cutting estimation wave scattering game of life video games interpolation people maths hexapawn puzzles golden spiral christmas card programming reddit bubble bobble braiding matrix of minors preconditioning go misleading statistics php platonic solids pac-man oeis mathslogicbot graph theory realhats graphs tennis the aperiodical menace gerry anderson logic machine learning bempp determinants statistics exponential growth plastic ratio trigonometry pythagoras matrix of cofactors pi approximation day cambridge ucl sport triangles matrix multiplication noughts and crosses squares mathsjam talking maths in public latex folding paper palindromes game show probability frobel christmas sobolev spaces

Archive

Show me a random blog post
▼ show ▼
© Matthew Scroggs 2012–2021