[BUG] Excess not labeled in plots

[FBumann]

Version Confirmation

• I have confirmed this bug exists on the latest release of FlixOpt

Bug Description

When plotting a node balance of a bus, the excess is not labeled properly.

Minimal Reproducible Example

import numpy as np
import pandas as pd

import flixopt as fx

timesteps = pd.date_range('2020-01-01', periods=3, freq='h')
flow_system = fx.FlowSystem(timesteps)

flow_system.add_elements(
    fx.Bus('District Heating'),
    fx.Effect('costs', '€', 'Cost', is_standard=True, is_objective=True),
    fx.Sink(
        'Heat Demand',
        inputs=[
            fx.Flow(label='Thermal Load', bus='District Heating', size=1, fixed_relative_profile=np.array([30, 0, 20]))
        ],
    ),
)

calculation = fx.FullCalculation('Simulation1', flow_system).do_modeling().solve(fx.solvers.HighsSolver(0.01, 60))
calculation.results['District Heating'].plot_node_balance()

Error Output

Solver Used

HiGHS (default)

Operating System

Mac OS 26

Python Version

3.13

Environment Info

<details>
 Replace this with your environment info 
</details>