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2.2 Line Graph

The Line Graph compares all values of all cars in one screen by grouping the same variables in vertical lines, the top of which represents the minimum, and the bottom of which represents the maximum value of the variable. The screen thus features as many vertical lines (bars) as there are variables. Each car is then represented by a poly-line that spans all bars from left to right.

Figure 1.6: Line Graph Frame
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When moving the mouse over the line graph, the poly-line closest to the mouse pointer is highlighted, and the corresponding car is displayed in an iconified version. Should more than one car be represented by the same segment of the highlighted poly-line, then moving the mouse pointer along the line toggles between all the different cars that are matched by this line.

Figure 1.7: Moving the mouse over the graph
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Clicking the mouse button when a car is highlighted, selects the car in all visualization frames. Selections work as toggles: clicking on an unselected car selects it; clicking on a selected car unselects it. All selected cars are marked with white lines.

Figure 1.8: Selected cars
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Each bar can serve as a reference by color coding all lines in a set color scheme. Once a bar's color button ( ) is selected, all lines are colored using the continuous scheme [magenta -> red -> yellow -> green -> cyan -> blue], starting from the minimum value and ending at the maximum value. Specifically, the color value is computed using the vertical position of where the lines cross the bar. All cars can now be tracked by the color that has been assigned at the reference bar. The user may learn from this representation how certain values (from the reference bar) branch into other variables.

A separate icon ( ) reverts the line graph into single-color mode.

Figure 1.9: Parameter MPG selected as a color reference
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Each bar can be moved horizontally by using the double-arrow icon ( ). This gives the user the ability to focus on a certain area of the graph. When a bar crosses another bar, the lines are adjusted so that each poly-line is still drawn from one side to the other without changing direction. This way, lines in-between different pairs of bars do not overlap.

Figure 1.10: Customized order of vertical parameter bars
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Because of the high density of lines in the original version of the graph, the zoom/pan tool proves to be useful in removing some of the clutter. The functionality of the graph when zooming up to 1000% remains the same as for the original graph: moving the mouse over the graph highlights the closest matching car.

Figure 1.11: Zoomed in at 400%
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When zooming in more than 1000%, the graph now displays iconified cars for every line on the screen.

Figure 1.12: Zoomed in at 1180%
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