## Brushing and linking Now I want to combine these two charts so I can explore across these data facets as well as within them. To do this, we first put both graphs in the same Observable cell. Then we can define brushing interactions between them using the local variables.
We use the following linking interactions: 1. Set selection types as a point selection
~~~ const selectType = vl.selectPoint("selected_pt") .on("click") // select on click .fields("Offense") // select all crimes of the same type .clear("dblclick") // double click to clear selection .toggle(true); // allow for multi-select (on by default) with shift-click ~~~ and a time scrubber interval selection. ~~~ const selectTimeBrush = vl.selectInterval().encodings("x"); ~~~

2. Then we need to identify where these selection operations are used. We do this with the `params` operator: ~~~ const crimeTrends = vl.markLine({ strokeWidth: 2 }) .params(selectTimeBrush) ~~~ and ~~~ const crimeBars = vl.markBar() .params(selectType) ~~~

3. Finally, we brush the related lines in the two timelines by setting the opacity of the other view marks to very faint: ~~~ vl.opacity().if(selectType, vl.value(0.99)).value(0.1) // if point selected, use opacity 0.99, otherwise 0.1 ~~~

But note while this supports brushing between these two charts, we also want to have a one-way filter applied, so that when we select a **subset** of the time, the **aggregate values in the bars are recalculated**. 4. To do this, we need to specify the filter operation in the bar chart: ~~~ .transform(vl.filter(selectTimeBrush)) ~~~ and now when we brush the timeline, we get recalculated values in the related bars. ### Constant vs dynamic scales When i filter the values in other view, the subset of data changes but the scale on the view axes remains **constant**. This is a design choice. It means that I have a fixed frame of reference but on the other hand I may miss some features in the data subset that have smaller values - try selecting one of the smaller bars to see how "flat" the timeline becomes.