embed_specviz.py

from qtpy.QtWidgets import QApplication
from specutils import Spectrum1D
from specviz.app import MainWindow

spec = Spectrum1D.read('gbt_1d.fits')
app = QApplication([''])
window = MainWindow()

# Load data into specviz
window.workspace.model.add_data(spec, name='banana')

# Get current plot widget
plot_widget = window.workspace.current_plot_window.plot_widget

# Add data to plot
item = window.workspace._model.item(2)
source_index = plot_widget.proxy_model.sourceModel().indexFromItem(item)
proxy_index = plot_widget.proxy_model.mapFromSource(source_index)
plot_widget.add_plot(proxy_index, visible=True, initialize=True)

window.show()
app.exec_()