Here are the metrics that provide you insightful information when viewed together.
Select one metric in one tree graph and the other in the second graph. Use drop-down option to choose metrics as below.
Spend vs Impressions: These are positively correlated metrics. This means as spends increase the impressions increase too. The areas on both charts should be proportional. If not, it indicates a channel that has higher Cost per Impression than the rest.
As an example, consider Google search below. The Spends are 6% of the total, but impressions are mere 1.7%. Such channels prove costly when the aim is visibility implying maximizing the impressions.
If the objective is to maximize the visits, orders or a mixed objective, then you can ignore the irregularities in this combination.
Impressions vs Visits: This again is a positively correlated pair of metrics. The more impressions Ads on a channel get the more clicks (or visits) it should drive. If the areas are not proportional, it means that the channel is costing more to get a click (cost per click is more than the other channels).
This matters when traffic is your primary objective. If conversions (or orders) is what you are looking for, then the traffic may not necessarily be an indicator. Some channels may drive traffic which is not relevant (the people coming do not purchase from your shop).
Visits vs Cart Sessions: Visits and cart sessions are proportional. Cart sessions are important since they clearly indicate the intent to buy.
If a channel has low transactions, but high cart sessions, it is still valuable for the orders since the people adding to the cart can be re-targeted.
Cart Sessions vs Orders: High intent channels, such as Google Search, ideally have a high cart session-to-transaction ratio. But, the reach of these channels is limited by the people searching for your keywords. Every business has an overall cart session to transaction ratio. If a particular channel has too many cart dropouts, you should rethink the strategy of this channel.
Spends to Orders: This graph provides you with the most converting channels. You should increase the spends on these channels till they reach an optimal amount. Since last-click attribution is used, the channels with high orders and low spends are not your new audience acquisition channels.
Spends to Revenue: If ROI matters to you, then this is the most important view. You can understand the channels that provide you with maximum ROI. Again, since last click attribution is used, the view can be biased towards the high intent channels.