My first impression is Euphorbia. Purple flowers is a stumper. This Euphorbia has red brown flowers and although the form and leaf arrangement is correct...the leaves look too herbaceous to be this plant.
When you break the stem is there a definitive milky sap? Do not get it on your hands or skin. Have you noticed this? Could you go break a chunk off using gloves? Euphorbia mellifera
How about Euphorbia austriaca? Euphorbia austriaca
Have you ever fertilized your landscape plants? The thing I just found is that Euphorbia shouldn't be fertilized UNTIL you see the yellowing bottom leaves. I am not so sure I like telling people how to recognize fertilizer deficiency before fertilizing. Sort of an advanced skill.
Nitrogen is a 'mobile chemical' (erroneously called nutrients). That means when the plant is finding no more Nitrogen in the soil it 'transfers' the nitrogen in the older leaves to the new leaves where that nitrogen is best used. The best leaves of the plant are then able to make food for the plant, they get the chemistry and water. The older leaves are abscised from the plant (yellowing first) and dumped so they don't take up carbohydrates and water needed by the best producers of food, the top leaves. This way the plant survives.
Have you used any fertilizer? Good old Osmocote 14-14-14 once a year would stop these beautiful plants yellowing at the bottom. Wear gloves when handling. Teach kids to never mess with plants until they are able to recite the scientific name!
The genus Euphorbia amazes me every time I get to look at the myriad forms! Cactus to shrubs to perennials to trees! The one thing that ties all those species together is Architectural. Like a robot plant. If your plant exudes a milky sap you have Euphorbia. The species is another matter. Purple flowers is another matter...