Not a web app, but OmniGraffle has sets of shapes and arrows that look handwritten and grungy. (And there are also a lot of sets available online made by third parties.)
There is no native functionality in Xcode for that. In the past I used Doxygen on Objective-C, but it does not support Swift. Looking at Jazzy, it does not seem to have that feature, but maybe it's hidden in some option.
I am not aware of any other tool. If you want to do it manually, I use Omnigraffle.
At this point you're much better off spending $99 on Sketch, it is much more prevalently used (it is literally everywhere and super streamlined for UI design). Alternately, Sketch is terrible for doing flows so I use Omnigraffle or (god forbid) PowerPoint or Google Slides (only if I need to collaborate with product owners).
I've used a variety of programs for this on a variety of operating systems. The best program I always come back to is OmniGraffle for the mac. It's not free, but I paid for this as a graduate student (neuroscience, lots of network diagrams) and it was honestly worth the $60 it costs with the student discount.
Another program to look into is GLE. It's a layout and graphing program that uses scripts instead of a UI (although there is a rudimentary UI). If you can get very proficient at the GLE language it's incredible. New data? Just replace your data file.
I use OmniGraffle for Mac. It's perfect for figures. It has a natural user interface and has deeper stuff (eg, you can draw lines to shapes then drag the shapes around and the lines stay connected). You can even drag in Latex equations and edit them in OmniGraffle by double clicking!
It's mac only, but I use OmniGraffle whenever I need to do quick and simple drawings. I don't often have to do Stage Plots, but it is definitely my software of choice when I do.
I don't use them much for work but since you asked, I want to put a plug in for Acorn and OmniGraffle - affordable, thoughtful products from small companies that seem to be run by really nice people.
I'm not sure there is a standard in Film & Television, in the same way Vectorworks is standard for Theatre / Concert lighting.
After a long search I settled on Omnigraffle. It's a diagraming program, but it has scale-accurate features, making it appropriate for professional lighting plots.
You can create stencil libraries of lighting fixtures. Measurements of your fixtures can be set, and when you drop them into your plot, the size will adjust to your working scale.
As a Rigging Gaffer, one thing I've found great is that I can draw in cable runs which helps estimate how much cable to order.
At $199 USD for the pro version, its far cheaper than Vectorworks and many other CAD programs, and the learning curve way less steep.
I'll do a full write-up post on it when I have some time.
https://www.omnigroup.com/omnigraffle/
Or
Both make great flow charts. Once a box or line is created you can easily move them around AND they stay connected. Great for working things out.
I do all my paperwork in Vectorworks Spotlight, but that's mainly because I spend so much time doing plans in it, that I am much quicker using it than having to learn and use another software package.
Many people use OmniGraffle (Mac only), which is what I first used before moving all-in to Vectorworks.
Check out Omnigraffle. I bought it years ago and while I don't use it for the same thing you're showing, it should be able to handle the job:
Have you tried OmniGraffle at all? it's great for doing super quick block diagrams. It's not a full-on CAD package by any means, but if you want to knock up a schematic easily and quickly it works great.
OmniGraffle is an awesome diagraming/drawing app (a la Visio), although at $50+ it is really more of a professional tool. (They also have a Mac version for $99). It's mainly focused on diagraming with shapes and lines but there is some support for free-form sketching as well.
Those are old-school software prices and the apps have a bit of a learning curve but in return you get full-featured and well-supported applications. For personal use it may be hard to swallow though.
Venn diagrams and flowcharts are more in order, tbh.
Someone with a penchant for Omnigraffle or Visio might find a Pulitzer Prize in it for themselves.