A two-dimensional scatter plot is a bivariate plot in which a comparison of 2 measures is presented, one measure along each axis. A scatter plot provides a useful exploratory method for understanding the correlation between the two measures (which may lead to conclusions about the dependence of one measure on the other).
In the following plot of data for the galaxy NGC 7531, a comparison is made between the relative velocity of parts of the galaxy with respect to Earth and the East/West position on the celestial sphere at which the measurement was made. In this data, measurements were taken at positions measured radially from the horizon through the center of the galaxy...specifically along one of seven slices, each at a given angle to the horizon. The measurements along the 133-degree slice are shown by "o" and those along the 43-degree slice are shown by "+". All other measurements are shown by "x".
It can be observed in the plot that, though the average relative velocity of the entire galaxy is approximately 1600 km/sec, the velocity of the eastern portion of the galaxy tends to be slower while the western portion tends to be faster. I.e., the galaxy appears to be spinning as it recedes from Earth. Further, the velocity markedly increases from East to West for measurements along the 133-degree slice while it generally descreases for measurements along the 43-degree slice. (These two slices being perpendicular to each other.) This indicates that the 133-degree angle more nearly matches the equator of the galaxy's rotation than the 43-degree angle, but is not the actual equator as other measurements exceed even these.
The data presented here is from R. Buta's study of the NGC 7531 galaxy, The Structure and Dynamics of Ringed Galaxies. III. Surface Photometry and Kinematics of the Ringed Nonbarred Spiral NGC 7531, which was published in 1987 as part of the Astrophysical Journal Supplement Series.
This data (and visual) is supplied with VisiCube as a sample project.
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