You need to study up on the graph editor. The colored lines show the different properties in an array. You are using Scale on a 2D layer so there will be a red and a green line. Change the layer to 3D and a blue line will be added to the display. There are two types of graphs you can edit. You choose between Speed and Value using the Choose Graph Type icon at the bottom of the graph editor. It is the second icon from the left. Scale always has colored lines. I can tell that you are editing the speed graph because it says 150%/sec in the legend at the top left corner of the graph. Sometimes it is easier to get the motion you want by editing the value graph. It is often helpful to have View Reference Graph turned on. You need to be familiar with the function of all fifteen icons at the bottom of the graph editor.
There is no gray line when looking at the scale property. You will get a gray line when a property does not have a link icon next to it like scale does. If you choose Position and look at the speed graph you will get a gray line. Separate Dimensions for Position (8th icon from the left) and the gray speed line will change to colored lines.
If the scale values are linked dragging any control handle will change both values at the same time. If the Link icon is off (It is just left of the values in the timeline) then Constrain Proportions is turned off and you can edit X, Y or Z values independently. If the layer is 3D Z scale does make a difference if you have moved the Z anchor point away from zero. Your scale values are linked but unequal:
so moving one control moves both. If you want to edit them separately unlink the scale values. If you want the lines to line up with each other at the keyframes then the values for X Y and Z (3D layer) must be equal.
There is a great deal more information available on the graph editor in the help files and community resources. You can find it by typing Graph Editor in the Search Help field at the top right corner of AE. If you are serious about your motion graphics and visual effects work Search Help should be one of your best friends.