Heartbeat Azure Monitoring the VM - piechart

You'll need to specify how you want your data aggregated before you plot it. In this case, try the following (rather verbose, to show the thinking process) query:

let healthyVMCountWindow = 30m; //time window we want to look at to identify how many healthy VMs we have
let healthyVMs = (){ //list healthy VMs over time window above
    Heartbeat
    | where TimeGenerated > ago(healthyVMCountWindow)
    | distinct Computer   
};
let totalVMCountWindow = 1d; //time window we want to look at to identify how many VMs we have in total
Heartbeat
| where TimeGenerated > ago(totalVMCountWindow)
| distinct Computer
| extend isHealthy = iff(Computer in (healthyVMs), "healthy", "unhealthy") //create a new column called "isHealthy" where we will classify our machines
| summarize count() by isHealthy
| render piechart 

Try it out on our demo environment, here!