# Visualization and Interaction¶

## Sankey Diagrams¶

### Requirements¶

- jupyter notebook
- ipysankeywidget

### Example¶

Code in this section uses the CE solar model

```
from solar import *
Vehicle = Aircraft(Npod=1, sp = False)
M = Mission(Vehicle, latitude=[20])
M.cost = M[M.aircraft.Wtotal]
sol = M.solve()
from gpkit.interactive.sankey import Sankey
Sankey(M).diagram(M.aircraft.Wtotal)
```

```
(objective) adds +1 to the sensitivity of Wtotal_Aircraft
(objective) is Wtotal_Aircraft [lbf]
Ⓐ adds +0.0075 to the overall sensitivity of Wtotal_Aircraft
Ⓐ is Wtotal_Aircraft <= 0.5*CL_Mission/Climb/AircraftDrag/WingAero_(0,)*S_Aircraft/Wing/Planform.2*V_Mission/Climb_(0, 0)**2*rho_Mission/Climb_(0, 0)
Ⓑ adds +0.0117 to the overall sensitivity of Wtotal_Aircraft
Ⓑ is Wtotal_Aircraft <= 0.5*CL_Mission/Climb/AircraftDrag/WingAero_(1,)*S_Aircraft/Wing/Planform.2*V_Mission/Climb_(0, 1)**2*rho_Mission/Climb_(0, 1)
```

### Explanation¶

Sankey
diagrams can be used to
visualize sensitivity structure in a model. A blue flow from a constraint to its parent
indicates that the sensitivity of the chosen variable (or of making the
constraint easier, if no variable is given) is negative; that
is, the objective of the overall model would improve if that variable’s
value were increased *in that constraint alone*. Red indicates a
positive sensitivity: the objective and the the constraint ‘want’ that
variable’s value decreased. Gray flows indicate a sensitivity whose
absolute value is below `1e-7`

, i.e. a constraint that is inactive for
that variable. Where equal red and blue flows meet, they cancel each
other out to gray.

### Usage¶

#### Variables¶

In a Sankey diagram of a variable, the variable is on the left with its final sensitivity; to the right of it are all constraints that variable is in.

##### Free¶

Free variables have an overall sensitivity of 0, so this visualization shows how the various pressures on that variable in all its constraints cancel each other out; this can get quite complex, as in this diagram of the pressures on wingspan:

```
Sankey(M).diagram(M.aircraft.b)
```

##### Fixed¶

Fixed variables can have a nonzero overall sensitivity. Sankey diagrams can how that sensitivity comes together:

Sankey(M).diagram(M[‘vgust’])

##### Equivalent Variables¶

If any variables are equal to the diagram’s variable (modulo some
constant factor; e.g. `2*x == y`

counts for this, as does `2*x <= y`

if the constraint is sensitive), they are found and plotted
at the same time, and all shown on the left. The constraints responsible
for this are shown next to their labels.

```
Sankey(M).sorted_by('constraints', 11)
```

#### Models¶

When created without a variable, the diagram shows the sensitivity of every named model to becoming locally easier. Because derivatives are additive, these sensitivities are too: a model’s sensitivity is equal to the sum of its constraints’ sensitivities. Gray lines in this diagram indicate models without any tight constraints.

```
Sankey(M).diagram(left=60, right=90, width=1050)
```

### Syntax¶

Code | Result |
---|---|

`s = Sankey(M)` |
Creates Sankey object of a given model |

`s.diagram(vars)` |
Creates the diagram in a way Jupyter knows how to present |

`d = s.diagram()` |
Don’t do this! Captures output, preventing Jupyter from seeing it. |

`s.diagram(width=...)` |
Sets width in pixels. Same for height. |

`s.diagram(left=...)` |
Sets top margin in pixels. Same for right, top. bottom. Use if the left-hand text is being cut off. |

`s.diagram(flowright=True)` |
Shows the variable / top constraint on the right instead of the left. |

`s.sorted_by("maxflow", 0)` |
Creates diagram of the variable with the largest single constraint
sensitivity. (change the `0` index to go down the list) |

```
s.sorted_by("constraints",
0)
``` |
Creates diagram of the variable that’s in the most constraints.
(change the `0` index to go down the list) |

## Plotting a 1D Sweep¶

Methods exist to facilitate creating, solving, and plotting the results of a single-variable sweep (see Sweeps for details). Example usage is as follows:

```
"Demonstrates manual and auto sweeping and plotting"
import matplotlib as mpl
mpl.use('Agg')
# comment out the lines above to show figures in a window
import numpy as np
from gpkit import Model, Variable, units
x = Variable("x", "m", "Swept Variable")
y = Variable("y", "m^2", "Cost")
m = Model(y, [y >= (x/2)**-0.5 * units.m**2.5 + 1*units.m**2, y >= (x/2)**2])
# arguments are: model, swept: values, posnomial for y-axis
sol = m.sweep({x: np.linspace(1, 3, 20)}, verbosity=0)
f, ax = sol.plot(y)
ax.set_title("Manually swept (20 points)")
f.show()
f.savefig("plot_sweep1d.png")
# arguments are: model, swept: (min, max, optional logtol), posnomial for y-axis
sol = m.autosweep({x: (1, 3)}, tol=0.001, verbosity=0)
f, ax = sol.plot(y)
ax.set_title("Autoswept (7 points)\nGuaranteed to be in blue region")
f.show()
f.savefig("plot_autosweep1d.png")
```

Which results in: