# Systems thinking diagrams in PlantUML

[[Systems thinking]] diagrams in [[PlantUML]].

Specifically a stock and flow diagram.

There's nothing built-in.

Someone [here](https://www.reddit.com/r/emacs/comments/prdsuo/use_emacs_for_systems_modeling_stockflow_diagrams/) and [here](https://junjizhi.com/all/experience/2021/09/19/emacs-systems-thinking/) has had a crack with Mermaid.

I think probably a deployment diagram looks the most hackable for this purpose in PlantUML.

Let's have a go.

Start with the bathtub example.

```plantuml
@startuml
left to right direction
cloud c1 as "..."
entity inflow
rectangle water as "water in tub"
entity outflow
cloud c2 as "..."

c1 -- inflow
inflow --> water
water -- outflow
outflow --> c2
@enduml
```

![[bathtub-stockflow.png]]

OK, that works OK.  You get a cloud out-of-the-box, and using `entity` to represent an icon for flow is good enough for now.  Could possibly use a sprite for that?

What about for something a bit more complex, with loops etc?

Let's try the coffee drinker example from [[Thinking in Systems]].

```plantuml
@startuml
rectangle stored as "stored energy in body"
entity expenditure as "energy expenditure"
cloud c1 as "..."
rectangle B {
entity mobilization as "metabolic mobilization of energy"
rectangle available as "energy available for work"
label discrepancy
label intake as "coffee intake"
}
label desired as "desired energy level"

stored -- mobilization
mobilization --> available
available -- expenditure
expenditure --> c1
available 0--> discrepancy
discrepancy 0--> intake
intake 0--> mobilization
desired 0--> discrepancy
@enduml
```

![[coffeedrinker-stockflow.png]]

Struggles a bit here with the feedback loop.  Hard to get it positioned in a useful place (I tried putting all the parts of the loop in a rectangle, but doesn't really work), and getting the 'B' inside the loop.

It's probably better to just sketch them, and then use something like the [NetLogo System Dynamics Modeler](https://ccl.northwestern.edu/netlogo/docs/systemdynamics.html) to actually implement them and check their dynamics.