Left justify the circuit in text visualizer

[maddy-tod]

What is the expected enhancement?

Currently when a circuit is displayed using the text drawer each gate in a circuit sits in its own column, as shown in the following circuit.

         ┌───┐          
q1_0: |0>┤ H ├──────────
         └───┘┌───┐     
q1_1: |0>─────┤ H ├─────
              └───┘     

It would be better if the circuits could be left justified, so the above circuit would appear as below.

         ┌───┐          
q1_0: |0>┤ H ├───────
         └───┘
         ┌───┐          
q1_1: |0>┤ H ├───────
         └───┘

It appears that circuits are already justified like this in the matplotlib drawer.

[maddy-tod]

In a circuit such as

circuit.h(qr1[1])
circuit.cx(qr1[0], qr1[3])
circuit.h(qr1[2])

The fully left justified circuit would be

q1_0: |0>──■───────
           │  ┌───┐
q1_1: |0>──┼──┤ H ├
           │  ├───┤
q1_2: |0>──┼──┤ H ├
         ┌─┴─┐└───┘
q1_3: |0>┤ X ├─────
         └───┘    

However this now no longer respects the order in which the gates were declared, and I think would be less helpful to users. I think it would be better to follow the style already used in the matplotlib drawer and produce a circuit where the gates are in their own columns, in the order they were declared. This will take up more space but would be clearer.

[ajavadia]

This is what matplotlib does. Do you mean this?

Or do you mean this?

                        
q3_0: |0>───────■───────
         ┌───┐  │       
q3_1: |0>┤ H ├──┼───────
         └───┘  │  ┌───┐
q3_2: |0>───────┼──┤ H ├
              ┌─┴─┐└───┘
q3_3: |0>─────┤ X ├─────
              └───┘     

You are right that the second one is more accurate in terms of gate order, but then that would not really be left justified. The reason I was proposing left justified is that it gives you a good sense of the circuit depth. Except when it is not visually appealing (in this example, circuit depth is 1, but you can't really draw it with a depth of 1, so you add one more column).

[taalexander]

The reason I was proposing left justified is that it gives you a good sense of the circuit depth

In practice on a device, gates are right justified (ie. gates are done as late as possible). The above could be confusing to users (I don't have a better solution, just wanted to note this).

[maddy-tod]

When I output that circuit to matplotlib the image I get is

which is the same as your second example. I get a different image because I was running it against a version of qiskit that includes my fix for #1685 - sorry for the confusion! It might make sense to have the circuits right justified if this is what is done on the devices, and this would also give a sense of the circuit depth.

[ajavadia]

Sure, I think there is merit to a right justified drawing as well. Left-justified is common in backend-agnostic circuits (textbooks, papers). Also the device schedule is something that is backend-specific and can change with different schedulers.

I think if we could have this as a drawing option in circuit.draw() it would be best (maybe justify='left'/'right' or schedule='asap'/'alap').