Sports statistics inspired “Starfish” visualisation of SAMI data
5 February 2016
Imagine you are a galaxy floating through space. You know more or less how luminous you are, how large, how massive. But how do you compare to your peers? Whether galaxies get status anxiety or not, this question is very interesting for astrophysicists, and the "Starfish" diagram now lets them know exactly how individual galaxies measure up with the Universe around them. The visualisation of individual targets and their properties is the "Where’s Waldo?" challenge in the time and age of very large astronomical surveys.
"Starfish" are, in essence, five-dimensional histograms. They plot a set of properties for an individual, along with the complete underlying statistical distribution of the sample from which these individuals are drawn. Each of the five histograms is pivoted about a central point of symmetry – this shape was decided to be optimal in user-testing. In that sense, "Starfish" approximate the function of violin plots but they extend the use case to precisely representing the place of the individual, rather than the distribution. The default option is to represent five attributes, but more can be accommodated.
The need for such a plotting technique came out of the SAMI Galaxy Survey, one of CAASTRO's flagship projects. When tasked with developing a data perusal interface, former CAASTRO Affiliate Dr Iraklis Konstantopoulos, at the Australian Astronomical Observatory (AAO), decided to rely solely on images, rather than text and numbers, to convey meaning. The user was invited to decide which galaxy dataset to download after perusing slices of hyper-spectral imaging. As a second layer of screening, Dr Konstantopoulos and the SAMI team provided a "Starfish" diagram to place the galaxy and its properties (redshift, stellar mass, surface brightness, effective radius, colour excess) in the context of the survey: is the galaxy exceptional in any way or is it regular? The users could review and decide which galaxies fit their experiment best.
Astrophysics is not the entire ballpark for these "Starfish". The inspiration for the design actually came from the visualisation of sports statistics in 1990's video games. There, the aptitude of an athlete in a range of indicators, such as speed, agility, strength etc, was captured by way of partially filling circles and pentagons. The "Starfish" design adds a representation in that every other player's skill level is included in the sample distribution and the individuals’ effective ranking among their peers is revealed at a glance.
"Starfish" can be used to represent any sample-individual relationship in datasets where the place of an individual needs to be visually quantified.