1) The left Y-Axes: Number of Tiles 2) The right Y-Axes: Meters Per Tile
The title implies the graph displays ballroom size and ceiling. Size can be measured both in terms of how many tiles there are as well as how big each tile is.
The data makes use of the "mean length per tile" - in other words, it uses the average length per tile.
Making sense of the Chart
Let's make sense of these 2 sets of data by picking one dot and thinking about what it means.
If we pick the top-left most red dot, we are looking at the data set that involves "ceiling height vs mean length per tile"
Each pair of dots is a ballroom.
Notice that for every red dot of a specific ceiling height, there is a corresponding black dot with that same exact ceiling height. This is true for all pieces of the data.
The question stem also points this out when it says "In the graph, each of the 22 Category C ballrooms is represented by two points arranged vertically...."
So this top-left most red dot basically says that
1) thisparticular ballroom is just over 10 meters in terms of ceiling height (X-axis).
2) The average length for each tile in the ballroom area was about 34 meters long.
Now, notice this top-left most red square actually corresponds with the black dot that is situated along the same imaginary vertical line. In this case, we are referring ot the bottom-left most black dot. So this same ballroom we were talking about has roughly
3) 55 tiles in the ballroom area.
So we can find 3 key pieces of information about each ballroom by laying the data in the above scatterplot style.
source: GMAT Pill - 2 y-axis scatterplot
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