Speaker impedance patterns - how to read them?

So here you go:
Most conventional cone speakers have a rising impedance as frequency goes up. What most speaker designers should do that do not want to impedance compensate their designs is rate the impedance based upon the operating range of the individual drivers. Most companies do fudge numbers, if a speaker that they are currently calling an 8 ohm speaker showed that dipped into 3 or 4 ohms, many tube aficionado’s would not buy them. A realistic example would be: if you have a 2 way and the woofer is rated at 8 ohms but actually varies from 4 at dc to 30 ohms at 4000 hz. Realistically that woofer will range from say 6 ohms at its 3db down point of say 35 hz or so to a 15 ohms at a crossover point of 2k.... So in the woofers operating range, it varies from 6 to 15 ohms. it is called an 8 ohm woofer.
Rising impedance can easily be controlled. So in the example above, that 30 ohms could be controlled to stay within 6 to 8 ohms. My last design is a 15 inch 2 way, it is 95 db in sensitivity and varies from 5 to 6 ohms at all frequencies and yes, even though a solid state amplifier handles rising impedance easier, it is still better for an amplifier to see a very consistent load and if your tubed amplifier had 4 ohm taps, it would much prefer a impedance compensated speaker. Normally overall, phase angles are tube amp killers as much as just impedance, a tubed amp lover should look at phase angles as well as impedance curves.
There are the basics with a very basic example. There are times that other scenario’s could happen, but the above is the most common for conventional drivers. I hope this helps, Tim

Actually Eric,  it is absolutely true.  You provided one driver.  I've measured hundreds of impedance curves not a couple.  
Yes,  I used a woofer as an example, but this happens in many mids, tweeters etc and their individual impedance curves do show up on a graph.  Of course, I've stated often on this site, overall, I do prefer and recommend impedance compensation networks. 
Unless you are describing some sort of ribbon/amt's which tend to have a very flat impedance curve. 

@perkri   
Your example of the ESL is an Electrostatic speaker. The op is trying to learn how to read impedance chart of a loud speaker.  Duke did give an excellent representation of how a solid state amp reacts to a speaker load vs a tubed amp. Which also shows why impedance compensation on a speaker design can be helpful. 
Your analysis of a crossover design can be true of how it effects a impedance curve, but this is normally due to how an inductance coil reacts to the EM field and natural inductance of the drivers. Depending on drivers,  it may be very beneficial to use a series or parallel design based on crossover point and what type of inductors are needed. But for the op's question and in general learning to read an impedance chart, my original response is less confusing for that purpose. 

Here you go op,  this should help.  Normally when I am building a speaker.  In design phase, I keep the crossovers external with the drivers in the box, this helps give me very accurate impedance and phase all along the design process.  
This person has done a pretty good job explaining as well as several examples that will help you get a good handle understanding. 
Speaker impedance curve explained with examples - Audio Judgement

@erik_squires 
Actually Erik,  no, I am not mixing up a driver vs a finished system.  I didn't say ALL speakers rise with frequency, Many drivers have a fairly smooth curve.   Here is a simple 2 way,  they show driver impedance as well as system impedance and add zobel.  Impedance compensation is the only thing that fixed these curves. You can see the curves.  
I'm sorry, but I've read through that blog that you reference, it give very basic info and doesn't show detail.  What you are saying about a driver vs that same driver within a system has minor merit, good compensation will affect the curve, but to say that a drivers individual impedance curve does not effect or control the system in the frequencies that it operates in wrong. This is not opinion, but fact.  I've done this hundreds of times. Too bad that we live so far apart,  I have an entire side of my basement shelved with drivers. We could do measurements together as well as do some system designs.  Its easy enough to show. I think that the miscommunication or (mixing up) is what a drivers curve looks like in free air vs what it looks like in its enclosure.  The curve can change quite a bit in an enclosure.  Again,  my goal here is not to argue, but to give the op some information on his inquirey.  I believe that I know of a good example.  I'll look around and if I find it,  I'll post. 
BR-1 manual.indd (daytonaudio.com)