What do you think

I don’t ha e a clue bluebell, but it adds something interesting to the leaf don’t you think?
I would turn your photo into a close up study and hang it on a wall somewhere!!!
Back yo your question, did you check the ground or pot to see if Ny signs of the culprit could be found?

I did check and no sign of anything untoward its so pretty the way it’s made a perfect line

Something has been eating my Canna leaves too, but not as neatly!

It is the right time of year for caterpillars to be out and about so...?

I’ve just had a thought, is the canna in the same soil as when you bought it? Could it have been lurking in the soil until now?
Caterpillars eggs are usually laid on leaves aren’t they, so presumably you would ha e seen them earlier?

An insect with OCD.

I asked the same question at a garden centre. They told me it is slugs. We have the same problem.

No it’s not in the same soil namsnanny I agree it’s the right time of years for caterpillars I just haven’t seen one at all anywhere in the garden
I would be very surprised at a slug liaise I have lots of plants eaten by them and they just chomp big old chomps nothing like this
I have lots of cabbage whites in the garden but I ve not seen them on the canna leaves on the poppies and foxgloves etc

It’s just so neat and measured that it intrigued me

I've never seen that before. Initially I thought it was the leaf cutter bee but the pics via Google show cuts only on the leaf edge. Googled a bit more and found:
Young Canna plants should be protected from slugs and snails which ignore full grown leaves but have a preference for the new shoots. A single nibble at this stage by a slug will cause a disfiguring row of holes as the leaf unfurls that will disfigure the leaf for the rest of the year.

That probably happened before the leaf unfurled, maybe a slug, who knows.

I don’t think that’s so venners although it should fit but the leaf is about two foot off the ground and I only noticed it this week I ve had slug pellets around it when it was growing and I don’t think any of the slugs or snails that came so eagerly to the pot survived the pellets ?

Just read your last sentence again and I m beginning to think you must be right as they say a neat row of holes which this is, so it must have been that, well I Never you live and learn Thanks for the google

Mystery solved!! Isnt nature grand?