Filter slider knobs should be blue

[pjcross]

In writing up the documentation, we use blue to represent the visible values in both the buttons and the slider range. However, the slider endpoints are drawn in gray, implying that values equal to the endpoints are not visible. Since the endpoints are included in the range, they should be blue too. This is especially a problem when inverting the selection. If one of the end regions consists of a single point, the impression from the slider is that all values are in the range shown by the bar, but really there is at least one point with the other end of the range. For example, in this filter, there is a point in the scatterplot with a value of 68. But by drawing the point in gray, it fades into the background and could come as a surprise to the user.