How does the steepness of the line on a speed graph relate to the objects speed

That depends on what is being graphed.

-- On a speed/time graph, the higher the speed, the higher the point on the line. 
A constant speed will be a horizontal piece of line.

-- On a distance/time graph, the higher the speed, the steeper the line is sloped.

-- On a temperature/time graph, the line on the graph is not related
to the object's speed at all.

-- etc.

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