The ideal meets the real. Simple and eloquent symbolism present in John Lennon's birth and derived charts.
The birth chart

The Sun is on the 7th cusp. L1, Mars is in the 7th cusp sign. A focus on “others” and the “world out there” is present for the native.
The Moon, RADIX L5 tightly trines Mars from house 11. Emotion, artistic expression and harmony among people (friendship) are also emphasized.
Mars is in the sign of Venus. Here is someone whose anger and power are directed pro-peace and not pro-war.
The recipe for an activist.
This is not a blind reading. I am reading the chart against the context. It is easy to do astrology like this. Nevertheless the symbolism is congruent, and entertaining.
Let's see the Solar Return and the Progressions for the period when his masterpiece "Imagine" was conceived.

First, let’s take notice of all the air sign presence in both the RADIX and the SR. Imagination is an expression of the air element.
Here we have the Moon not far from the RADIX Moon right inside the 7th cusp; in the most humane of the signs. Being a Saturn sign we would have boundaries, direction, discipline, respect and restraint; ideally…
We could stop here: “imagine all the people living as one”. Imagination of an emotional type with a Saturnine flavor.
That is the ideal. But there is more to it. Let’s see what the charts say about the real and whether the ideal and the real match one another.
The SR Moon is conjunct Venus, RADIX L7, on the 5th cusp by antiscion. Venus trines the RADIX 5th cusp.
How is his imagination going to be expressed? In a musical creation. Cool. This is a special year. The Sun is angular. Things will happen.
Was his ideal something that was possible to realize?
Well, not “all” the people are into peace and love. And Lennon knew that. That is the very reason he wrote the lyrics to begin with. So, in the SR we have L7, Saturn in an opposition with Venus and trining Mars. That is: people against people having an impact on Lennon and his creation.
Yes, imagine all the people living as one. That would be a conjunction. The reality is that we have an opposition. Astrology does not get more eloquent that this.
Let’s take a peek at the secondary progressions as they drive the message further.

Venus, “other people” (RADIX L7), is conjunct progressed Fortuna and the North Node. The Moon is conjunct Mars by antiscion. Again, delivering the message that his heart, his creative expression and other people are deeply and strongly connected during this period in his life. A beautiful human expression.
Progressed Mars on the star Spica is a nice cherry on the cake for the spiritual slant the situation contains and promotes.
Yet, Venus and Fortuna are conjunct the star Vindemiatrix telling us that all those lofty emotions, that hope and desire for peace are overstretched and the ideal of “all the people living as one” is beyond reach.
If that was not enough, his progressed Arabic Part of Courage is on the Pleiades, reinforcing the fact that despite his expression being “Right Action” it is being expressed in an indeed imaginary, hazy, way. Too close to Alcyone for comfort. This is not about happiness at all, this is about tears and disappointment.
Lennon’s picture on the album cover looks like a picture of this progression chart to me. This is not a happy situation. It is extremely emotional but it is hopelessly unrealizable. And John knows and feels it. It is plainly sad.